minor #21206 [Lock] Document what values to use to activate the Lock component NullStore and InMemoryStore (hmeknassi)

This PR was submitted for the 7.3 branch but it was merged into the 7.2 branch instead.

Discussion
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[Lock] Document what values to use to activate the Lock component NullStore and InMemoryStore

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.

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Commits
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0cb7c16 Update lock.rst

Author: javiereguiluz

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