DOC: Improve documentation on http-request set-src

This patch adds more explanation on how to use "http-request set-src"
and a link to "option forwardfor".

This patch can be applied to all previous version starting at 1.6

Reviewed-by: Tim Duesterhus <tim@bastelstu.be>

doc/configuration.txt

@@ -5114,16 +5114,23 @@ http-request set-src <expr> [ { if | unless } <condition> ]
   This is used to set the source IP address to the value of specified
   expression. Useful when a proxy in front of HAProxy rewrites source IP, but
   provides the correct IP in a HTTP header; or you want to mask source IP for
-  privacy.
+  privacy. All subsequent calls to "src" fetch will return this value
+  (see example).
 
   Arguments :
     <expr>  Is a standard HAProxy expression formed by a sample-fetch followed
             by some converters.
 
+  See also "option forwardfor".
+
   Example:
     http-request set-src hdr(x-forwarded-for)
     http-request set-src src,ipmask(24)
 
+    # After the masking this will track connections
+    # based on the IP address with the last byte zeroed out.
+    http-request track-sc0 src
+
   When possible, set-src preserves the original source port as long as the
   address family allows it, otherwise the source port is set to 0.