#27345 Added more info about the use of _id and index restrictions

Author: Joe Bennett

components/lock.rst

@@ -466,6 +466,7 @@ method uses the Memcached's ``flush()`` method which purges and removes everythi
 MongoDbStore
 ~~~~~~~~~~~~
 
+The locked resouce name is indexed in the ``_id`` field of the lock collection.
 An indexed field's value in MongoDB can be a maximum of 1024 bytes in length
 inclusive of structural overhead.