c-user: Split up semaphore manager

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

Update #3993.

c-user/semaphore/introduction.rst

@@ -0,0 +1,25 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+.. Copyright (C) 1988, 2008 On-Line Applications Research Corporation (OAR)
+
+Introduction
+============
+
+The semaphore manager utilizes standard Dijkstra
+counting semaphores to provide synchronization and mutual
+exclusion capabilities.  The directives provided by the
+semaphore manager are:
+
+- :ref:`rtems_semaphore_create`
+
+- :ref:`rtems_semaphore_ident`
+
+- :ref:`rtems_semaphore_delete`
+
+- :ref:`rtems_semaphore_obtain`
+
+- :ref:`rtems_semaphore_release`
+
+- :ref:`rtems_semaphore_flush`
+
+- :ref:`rtems_semaphore_set_priority`