Clarify storage backend usage

Some more text describing the behaviour on the various storage backend
+ some reformatting for readability

doc/sphinx/reference/varnishd.rst

@@ -156,10 +156,16 @@ Storage Types
 The following storage types are available:
 
 malloc[,size]
-      Storage for each object is allocated with malloc(3).
 
-      The size parameter specifies the maximum amount of memory varnishd will allocate.  The size is assumed to
-      be in bytes, unless followed by one of the following suffixes:
+      Malloc is a memory based backend. Each object will be allocated
+      from memory. If your system runs low on memory swap will be
+      used. Be aware that the size limitation only limits the actual
+      storage and that approximately 1k of memory per object will be
+      used for various internal structures.
+
+      The size parameter specifies the maximum amount of memory
+      varnishd will allocate.  The size is assumed to be in bytes,
+      unless followed by one of the following suffixes:
 
       K, k    The size is expressed in kibibytes.
 
@@ -169,16 +175,24 @@ malloc[,size]
 
       T, t    The size is expressed in tebibytes.
 
-      The default size is unlimited.
+      The default size is unlimited. 
+
+      Mallocs performance is bound by memory speed so it is very fast. 
 
 file[,path[,size[,granularity]]]
-      Storage for each object is allocated from an arena backed by a file.  This is the default.
 
-      The path parameter specifies either the path to the backing file or the path to a directory in which
-      varnishd will create the backing file.  The default is /tmp.
+      The file backend stores objects in memory backed by a file on
+      disk with mmap. This is the default storage backend and unless
+      you specify another storage this one will used along with
+      Transient storage.
 
-      The size parameter specifies the size of the backing file.  The size is assumed to be in bytes, unless fol‐
-      lowed by one of the following suffixes:
+      The path parameter specifies either the path to the backing file
+      or the path to a directory in which varnishd will create the
+      backing file.  The default is /tmp.
+
+      The size parameter specifies the size of the backing file.  The
+      size is assumed to be in bytes, unless fol‐ lowed by one of the
+      following suffixes:
 
       K, k    The size is expressed in kibibytes.
 
@@ -188,21 +202,29 @@ file[,path[,size[,granularity]]]
 
       T, t    The size is expressed in tebibytes.
 
-      %       The size is expressed as a percentage of the free space on the file system where it resides.
+      %       The size is expressed as a percentage of the free space on the
+              file system where it resides.
 
       The default size is 50%.
 
-      If the backing file already exists, it will be truncated or expanded to the specified size.
+      If the backing file already exists, it will be truncated or
+      expanded to the specified size.
+
+      Note that if varnishd has to create or expand the file, it will
+      not pre-allocate the added space, leading to fragmentation,
+      which may adversely impact performance.  Pre-creating the
+      storage file using dd(1) will reduce fragmentation to a minimum.
 
-      Note that if varnishd has to create or expand the file, it will not pre-allocate the added space, leading
-      to fragmentation, which may adversely impact performance.  Pre-creating the storage file using dd(1) will
-      reduce fragmentation to a minimum.
+      The granularity parameter specifies the granularity of
+      allocation.  All allocations are rounded up to this size.  The
+      size is assumed to be in bytes, unless followed by one of the
+      suffixes described for size except for %.
 
-      The granularity parameter specifies the granularity of allocation.  All allocations are rounded up to this
-      size.  The size is assumed to be in bytes, unless followed by one of the suffixes described for size except
-      for %.
+      The default size is the VM page size.  The size should be
+      reduced if you have many small objects.
 
-      The default size is the VM page size.  The size should be reduced if you have many small objects.
+      File performance is typically limited by the write speed of the
+      device, and depending on use, the seek time.
 
 persistent,path,size {experimental}
 
@@ -241,38 +263,47 @@ Transient Storage
 Management Interface
 --------------------
 
-If the -T option was specified, varnishd will offer a command-line management interface on the specified address
-and port.  The recommended way of connecting to the command-line management interface is through varnishadm(1).
+If the -T option was specified, varnishd will offer a command-line
+management interface on the specified address and port.  The
+recommended way of connecting to the command-line management interface
+is through varnishadm(1).
 
 The commands available are documented in varnish(7).
 
 Run-Time Parameters
 -------------------
 
-Runtime parameters are marked with shorthand flags to avoid repeating the same text over and over in the table
-below.  The meaning of the flags are:
+Runtime parameters are marked with shorthand flags to avoid repeating
+the same text over and over in the table below.  The meaning of the
+flags are:
 
 experimental
-      We have no solid information about good/bad/optimal values for this parameter.  Feedback with experience
-      and observations are most welcome.
+      We have no solid information about good/bad/optimal values for
+      this parameter.  Feedback with experience and observations are
+      most welcome.
 
 delayed
-      This parameter can be changed on the fly, but will not take effect immediately.
+      This parameter can be changed on the fly, but will not take
+      effect immediately.
 
 restart
-      The worker process must be stopped and restarted, before this parameter takes effect.
+      The worker process must be stopped and restarted, before this
+      parameter takes effect.
 
 reload
       The VCL programs must be reloaded for this parameter to take effect.
 
-Here is a list of all parameters, current as of last time we remembered to update the manual page.  This text is
-produced from the same text you will find in the CLI if you use the param.show command, so should there be a new
-parameter which is not listed here, you can find the description using the CLI commands.
+Here is a list of all parameters, current as of last time we
+remembered to update the manual page.  This text is produced from the
+same text you will find in the CLI if you use the param.show command,
+so should there be a new parameter which is not listed here, you can
+find the description using the CLI commands.
 
 Be aware that on 32 bit systems, certain default values, such as
-workspace_client (=16k), thread_pool_workspace (=16k), http_resp_size (=8k),
-http_req_size (=12k), gzip_stack_buffer (=4k) and thread_pool_stack (=64k) are
-reduced relative to the values listed here, in order to conserve VM space.
+workspace_client (=16k), thread_pool_workspace (=16k), http_resp_size
+(=8k), http_req_size (=12k), gzip_stack_buffer (=4k) and
+thread_pool_stack (=64k) are reduced relative to the values listed
+here, in order to conserve VM space.
 
 .. include:: params.rst