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

* doc/api-messages.texi (Message Handling): Add description for
  `message-reply-success' and for
  `message-channel-request-reply-success' procedure.
doc/api-sessions.texi (Sessions): Add general description.

* doc/api-sessions.texi (Sessions): Add general description of a
  session and its relationships with channels.
Merge branch 'master' into tests-implementation

Conflicts:
	ChangeLog
tests/auth.scm: Remove.

* tests/auth.scm: Remove.
* Makefile.am: Likewise.
tests/session.scm, tests/server.scm: Update comments.