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/software-requirements.rst b/docs/developer/software-requirements.rst
index ec6c2e4..700c32d 100644
--- a/docs/developer/software-requirements.rst
+++ b/docs/developer/software-requirements.rst
@@ -23,7 +23,7 @@
          (Note: the compiler part of the OP-TEE build environment works too.)
        * For `linux-pc` environment use the native version of GCC available in your Linux distribution.
 
-To build the documentation, please refer to :ref:`Building Documentation`.
+To build the documentation, please refer to :ref:`Documentation Build Instructions`.
 
 .. todo:: Add info on commit validation dependencies (checkpatch, uncrustify, etc...)