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/uml/ProtectedProxyDeploymentDiagram.puml b/docs/developer/service-descriptions/uml/ProtectedProxyDeploymentDiagram.puml
new file mode 100644
index 0000000..a6d5c14
--- /dev/null
+++ b/docs/developer/service-descriptions/uml/ProtectedProxyDeploymentDiagram.puml
@@ -0,0 +1,25 @@
+'-------------------------------------------------------------------------------
+' Copyright (c) 2021, Arm Limited and Contributors. All rights reserved.
+'
+' SPDX-License-Identifier: BSD-3-Clause
+'
+'-------------------------------------------------------------------------------
+
+@startuml
+
+node crypto_sp {
+	[its_frontend:0] -> [secure_storage_client:0]
+}
+
+node protected_store_sp {
+	[secure_storage_provider:1] -> [secure_storage_client:1]
+}
+
+node sel1_sp {
+	[secure_storage_provider:2] -> [optee_store:2]
+}
+
+[secure_storage_client:0] -> [secure_storage_provider:1]
+[secure_storage_client:1] -> [secure_storage_provider:2]
+
+@enduml
\ No newline at end of file