doc/api-messages.texi (Message Handling): Improve doc.

* doc/api-messages.texi (Message Handling): Add general
  description of a message.  Improve description of
  `message-reply-success' procedure.
doc/api-auth.texi (Auth): Add description of `(ssh auth)' module.

* doc/api-auth.texi (Auth): Add general description of `(ssh
  auth)' module.
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.
README (Distribution): Update.

* README (Distribution): Add information about documentation.  Fix
  a typo.
README: Update.
tests/server-client.scm ("message-get-type"): New TC.
src/message.scm (message?): Export.

* src/message.scm (message?): Export.
* NEWS: Update.
Merge branch 'master' into tests-implementation

Conflicts:
	ChangeLog
tests/server-client.scm ("server-message-get"): Improve TC.