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>
(cherry picked from commit 56e3120f9ee0db8de166f5e6c9cf2ce2fc4c2364)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 5a8526222f2a905d5fae66586e83b8b7d53640ae)
Signed-off-by: Willy Tarreau <w@1wt.eu>
1 file changed