DOC: config: Fix description of options about HTTP connection modes

DOC: config: Fix description of options about HTTP connection modes

Since the HTX, the decription of options about HTTP connection modes is
wrong. In fact, it is worst, all the documentation about HTTP connection
mode is wrong. But only options will be updated for now to be backported.

So, documentation of "option httpclose", "option "http-keep-alive", "option
http-server-close" and "option "http-pretend-keepalive" was reviewed. First,
it is specify these options only concern HTT/1.x connections. Then, the
descriptions were updated to reflect the HTX implementation.

The main changes concerns the fact that server connections are no longer
attached to client connections. The connection mode on one side does not
affect the connection mode on the other side. It is especially true for
t"option httpclose". For client connections, only the frontend option is
considered and for server ones, both frontend and backend options are
considered.

This patch should be backported as far as 2.2.