DOC: http: document 413 response code

DOC: http: document 413 response code

Considering that we only use a "413 Payload Too Large" response in a single
situation with a specific config toogle (h1-accept-payload-with-any-method),
add some text to make it easier to find.

Should be backported to 2.6.

Link: https://github.com/cbonte/haproxy-dconv/issues/46
Link: https://discourse.haproxy.org/t/haproxy-error-413-paylod-too-large/9831/3