Use code litterals for the grouping modes

doc/sphinx/reference/vsl-query.rst

@@ -46,7 +46,7 @@ be 0.
 
 The grouping modes are:
 
-* Session
+* ``session``
 
   All transactions initiated by a client connection are reported
   together. Client connections are open ended when using HTTP
@@ -55,21 +55,21 @@ The grouping modes are:
   incomplete session will be reported. Non-transactional data (vxid
   == 0) is not reported.
 
-* Request
+* ``request``
 
   Transactions are grouped by request, where the set will include the
   request itself as well as any backend requests or ESI-subrequests.
   Session data and non-transactional data (vxid == 0) is not
   reported.
 
-* VXID
+* ``vxid``
 
   Transactions are not grouped, so each vxid is reported in it's
   entirety. Sessions, requests, ESI-requests and backend requests are
   all reported individually. Non-transactional data is not reported
   (vxid == 0). This is the default.
 
-* Raw
+* ``raw``
 
   Every log record will make up a transaction of it's own. All data,
   including non-transactional data will be reported.