c-user: Split up message manager

This makes it easier to automatically generate parts of the manager
documentation in the future.

Update #3993.

c-user/message/introduction.rst

@@ -0,0 +1,28 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+.. Copyright (C) 1988, 2008 On-Line Applications Research Corporation (OAR)
+
+Introduction
+============
+
+The message manager provides communication and synchronization capabilities
+using RTEMS message queues.  The directives provided by the message manager
+are:
+
+- :ref:`rtems_message_queue_create`
+
+- :ref:`rtems_message_queue_ident`
+
+- :ref:`rtems_message_queue_delete`
+
+- :ref:`rtems_message_queue_send`
+
+- :ref:`rtems_message_queue_urgent`
+
+- :ref:`rtems_message_queue_broadcast`
+
+- :ref:`rtems_message_queue_receive`
+
+- :ref:`rtems_message_queue_get_number_pending`
+
+- :ref:`rtems_message_queue_flush`