Add secure storage service docs

Adds a description of the secure storage service.  Also updates and
fixes a few other documentation niggles.

Signed-off-by: Julian Hall <julian.hall@arm.com>
Change-Id: Ie212cc12344f71a2ef2f9d683a967b0433acf866
diff --git a/docs/developer/writing-documentation.rst b/docs/developer/writing-documentation.rst
index 850d9cd..eb10449 100644
--- a/docs/developer/writing-documentation.rst
+++ b/docs/developer/writing-documentation.rst
@@ -1,4 +1,4 @@
-Writing documentation
+Writing Documentation
 =====================
 
 |TS| is documented using `Sphinx`_, which in turn uses Docutils and `Restructured Text`_ (|REST| hereafter).