DOC: tcp-rules: Refresh details about L7 matching for tcp-request content rules

DOC: tcp-rules: Refresh details about L7 matching for tcp-request content rules

Because the parsing of HTTP message is now performed in the HTTP multiplexers,
the content is immediatly available when "tcp-request content" rules are
evaluated for an HTTP frontend. So, it is a good idea to make the documentation
explicit on this point. In addition, because in all cases, the parsing is
already performed, there is no reason to still use "tcp-request content" rules
based on L7 matching, although it is still valid. The recommended way is to use
"http-request" rules instead. Again, it is a good idea to update the
documentation on this point.