[Lock] Add reference tip for the InMemoryStore lock

Author: 94noni

components/lock.rst

@@ -667,6 +667,14 @@ PHP process is terminated::
     Zookeeper does not require a TTL as the nodes used for locking are ephemeral
     and die when the PHP process is terminated.
 
+InMemoryStore
+~~~~~~~~~~~~~
+
+.. tip::
+
+    An ``InMemoryStore`` is available for saving lock in memory during a process,
+    and can be useful for testing.
+
 Reliability
 -----------