Clarify %r formatter wrt multiple Host headers

Also clarify %{X}i and %{X}o wrt multiple X headers.

Fixes #2190.

doc/sphinx/reference/varnishncsa.rst

@@ -98,7 +98,8 @@ Supported formatters are:
   total bytes sent to the backend.
 
 %{X}i
-  The contents of request header X.
+  The contents of request header X. If the header appears multiple times
+  in a single transaction, the last occurrence is used.
 
 %l
   Remote logname. Always '-'.
@@ -107,7 +108,8 @@ Supported formatters are:
   Request method. Defaults to '-' if not known.
 
 %{X}o
-  The contents of response header X.
+  The contents of response header X. If the header appears multiple
+  times in a single transaction, the last occurrence is used.
 
 %O
   In client mode, total bytes sent to client.  In backend mode, total
@@ -178,8 +180,7 @@ Supported formatters are:
     of fields, and only the nth part of the record will be matched
     against. Fields start counting at 1. Defaults to '-' when the tag
     is not seen, or when the field is out of bounds. If a tag appears
-    several times in a single transaction, only the first occurrence
-    is used.
+    multiple times in a single transaction, the first occurrence is used.
 
 SIGNALS
 =======
@@ -195,6 +196,8 @@ NOTES
 
 The %r formatter is equivalent to "%m http://%{Host}i%U%q %H". This
 differs from apache's %r behavior, equivalent to "%m %U%q %H".
+Furthermore, when using the %r formatter, if the Host header appears
+multiple times in a single transaction, the first occurrence is used.
 
 EXAMPLE
 =======