doc/: Improve documentation of the Guile-SSH modules.

* doc/api-messages.texi (Messages): Improve documentation of the
  module.
* doc/api-servers.texi (Servers): Likewise.
* doc/api-sessions.texi (Sessions): Likewise.
* doc/api-keys.texi (Keys): Likewise.
* doc/api-version.texi (Version): Likewise.
* doc/api-channels.texi (Channels): Likewise.  Add references.
  Improve documentation of `make-channel' and `channel-open-session'
  procedure.