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/docs-build.rst b/docs/developer/docs-build.rst
index 3ae4224..9f36e64 100644
--- a/docs/developer/docs-build.rst
+++ b/docs/developer/docs-build.rst
@@ -1,5 +1,5 @@
-Building Documentation
-======================
+Documentation Build Instructions
+================================
 
 To create a rendered copy of this documentation locally you can use the
 `Sphinx`_ tool to build and package the plain-text documents into HTML-formatted
@@ -57,7 +57,7 @@
 
    . ~/sphinx-venv/bin/activate
    make clean
-   make SPHINXOPTS=-W html
+   make
    deactivate
 
 Output from the build process will be placed in: