Docs: Update source structure for 'spm/common'

Update the description for 'spm/common' in details.

Change-Id: Ibd4d7a7ea6d217c8c8d8eddfca164be79cc6c3ae
Signed-off-by: Summer Qin <summer.qin@arm.com>
diff --git a/docs/design_documents/source_structure.rst b/docs/design_documents/source_structure.rst
index 957b7a3..208c7bf 100644
--- a/docs/design_documents/source_structure.rst
+++ b/docs/design_documents/source_structure.rst
@@ -145,7 +145,10 @@
 Folder name                         Purpose
 =================================== ===========================================
 include/\*                          SPM public headers.
-common/\*                           SPM common logic complies PSA-FF-M.
+common/\*                           SPM common logic complies with PSA-FF-M and
+                                    its necessary supporting functionalities,
+                                    such as the runtime API and the thread
+                                    operation, etc.
 cmsis_psa/\*                        CMSIS implementation for PSA-FF-M SPM. [1]
 cmsis_func/\*                       The library model implementation. [2]
 \*                                  Implementation sources.