Update lock.rst

Adds a clarification in the Lock component documentation about the the values to use for the env variable LOCK_DSN in order to activate the NullStore and the InMemoryStore.

Author: hmeknassi

components/lock.rst

@@ -405,7 +405,8 @@ Store                                                       Scope   Blocking  Ex
 
     Symfony includes two other special stores that are mostly useful for testing:
     ``InMemoryStore``, which saves locks in memory during a process, and ``NullStore``,
-    which doesn't persist anything.
+    which doesn't persist anything. To activate either one, simply set the ``LOCK_DSN`` to
+    ``null`` or ``in-memory``.
 
 .. versionadded:: 7.2