docs/cmocka: Fill out documentation for cmocka

Write documentation for cmocka.

Signed-off-by: Matteo Golin <matteo.golin@gmail.com>

Documentation/applications/testing/cmocka/index.rst

@@ -1,3 +1,38 @@
 ====================
 ``cmocka`` libcmocka
 ====================
+
+cmocka is:
+    an elegant unit testing framework for C with support for mock objects. It
+    only requires the standard C library, works on a range of computing
+    platforms (including embedded) and with different compilers.
+
+The cmocka application in the NuttX apps is an adaptation of cmocka into the
+NuttX build system.
+
+You can find out more about how to use the cmocka library on the official
+`cmocka website <https://cmocka.org/index.html>`_.
+
+Interface Notes
+---------------
+
+The cmocka library header requires that several other standard header files be
+included prior to it. Your include statement should look like this:
+
+.. code:: c
+
+   #include <setjmp.h>
+   #include <stdarg.h>
+   #include <stddef.h>
+   #include <stdint.h>
+
+   #include <cmocka.h>
+
+Build options
+-------------
+
+* ``CONFIG_TESTING_CMOCKA_PROG``: If enabled, a `cmocka` test binary will be
+  included in your image.
+
+* ``CONFIG_TESTING_CMOCKA_LEAKDETECT``: Cmocka can check for memory leaks.
+  Intended to be used in a local environment.