[#2403] Editorial review of documentation about hook libraries in the ARM

doc/sphinx/arm/hooks-cb-mysql.rst

@@ -0,0 +1,9 @@
+.. _hooks-cb-mysql:
+
+``mysql_cb``: MySQL Configuration Backend
+=========================================
+
+This hook library works in conjunction with the ``cb_cmds`` library to
+implement the API to create, read, update, and delete (CRUD) the
+configuration in a MySQL database. Please see :ref:`hooks-cb-cmds`
+for more details.