doc: Add section on microkernel application use of fibers

Adds a short section to make clear that microkernel applications
can utilize fibers, and can even use them with a few microkernel
object types. Revises some link names to account for the fact that
there are now 2 sections describing fiber services.

Change-Id: I5960aea8b3959bb4a383f919882419effff5917d
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>

doc/kernel/microkernel/microkernel.rst

@@ -11,6 +11,7 @@ nanokernel applications.
    :maxdepth: 1
 
    microkernel_tasks
+   microkernel_fibers.rst
    microkernel_timers
    microkernel_memory
    microkernel_synchronization

doc/kernel/microkernel/microkernel_fibers.rst

@@ -0,0 +1,21 @@
+.. _microkernel_fibers:
+
+Fiber Services
+##############
+
+Concepts
+********
+
+A fiber is a lightweight, non-preemptible thread of execution that implements
+a portion of an application's processing. It is is normally used when writing
+device drivers and other performance critical work.
+
+A microkernel application can use all of the fiber capabilities that are
+available to a nanokernel application; for more information see
+:ref:`Nanokernel Fiber Services <nanokernel_fibers>`.
+
+While a fiber often uses one or more nanokernel object types to carry
+out its work, it can also interact with microkernel events and semaphores
+to a limited degree. For example, a fiber can signal a task by giving a
+microkernel semaphore, but it cannot take a microkernel semaphore. For more
+information see :ref:`microkernel_events` and :ref:`microkernel_semaphores`.

doc/kernel/nanokernel/nanokernel_fibers.rst

@@ -1,4 +1,4 @@
-.. _fiber_services:
+.. _nanokernel_fibers:
 
 Fiber Services
 ##############

doc/kernel/overview/kernel_fundamentals.rst

@@ -69,8 +69,8 @@ is limited to a single task.
 
 Related sections:
 
-* :ref:`Fiber Services <fiber_services>`
-* :ref:`Task Services <tasks>`
+* :ref:`Nanokernel Fiber Services <nanokernel_fibers>`
+* :ref:`Microkernel Task Services <microkernel_tasks>`
 
 Interrupts
 **********