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/service-descriptions/index.rst b/docs/developer/service-descriptions/index.rst
index 6f710a4..6762238 100644
--- a/docs/developer/service-descriptions/index.rst
+++ b/docs/developer/service-descriptions/index.rst
@@ -6,6 +6,7 @@
     :caption: Contents:
 
     crypto-service-description
+    secure-storage-service-description
 
 --------------