doc: remove @return doc for void functions in sample code

For functions returning nothing, there is no need to document
with @return, as Doxgen complains about "documented empty
return type of ...".

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

doc/development_process/documentation.rst

@@ -99,7 +99,6 @@ requirements, we use the ``satisfy`` alias in doxygen::
     *
     * @param sem Address of the semaphore.
     *
-    * @return N/A
     * @satisfy{@req{015}}
     */
     __syscall void k_sem_give(struct k_sem *sem);