doc: Add missing Usage section title

Change-Id: I2533bd7f16ad1e3ba44f9c0c282067dc82d33c26
Signed-off-by: Peter Mitsis <peter.mitsis@windriver.com>

doc/kernel/nanokernel/nanokernel_fibers.rst

@@ -142,6 +142,9 @@ code that will take some time, periodically call
 :cpp:func:`fiber_yield()`. Multi-threading using blocking fibers is
 effective in coding hard real-time applications.
 
+Usage
+*****
+
 Example: Starting a Fiber from a Task
 =====================================
 

doc/kernel/nanokernel/nanokernel_fifos.rst

@@ -26,6 +26,9 @@ without waiting on the data to be stored.
    block as a link pointer to the next item. The size of the WORD
    depends on the platform and can be 16 bit, 32 bit, etc.
 
+Usage
+*****
+
 Example: Initializing a Nanokernel FIFO
 =======================================
 

doc/kernel/nanokernel/nanokernel_synchronization.rst

@@ -32,6 +32,9 @@ Thus after n 'give' operations a semaphore can 'take' n times without
 pending. If a second context waits for the same semaphore object, the
 first context is lost and never wakes up.
 
+Usage
+*****
+
 Example: Initializing a Nanokernel Semaphore
 ============================================
 

doc/kernel/nanokernel/nanokernel_timers.rst

@@ -74,6 +74,8 @@ other work.
    the system clock, as well as the higher precision hardware clock,
    without using a nanokernel timer.
 
+Usage
+*****
 
 Example: Initializing a Nanokernel Timer
 ========================================