Documentation: stable: Document alternative for referring upstream commit hash

Additionally to the "commit <sha1> upstream." variant, "[ Upstream
commit <sha1> ]" is used as well as alternative to refer to the upstream
commit hash.
Signed-off-by: Salvatore Bonaccorso <carnil@debian.org>
Link: https://lore.kernel.org/r/20220901184328.4075701-1-carnil@debian.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/process/stable-kernel-rules.rst

@@ -97,6 +97,12 @@ text, like this:
 
     commit <sha1> upstream.
 
+or alternatively:
+
+.. code-block:: none
+
+    [ Upstream commit <sha1> ]
+
 Additionally, some patches submitted via :ref:`option_1` may have additional
 patch prerequisites which can be cherry-picked. This can be specified in the
 following format in the sign-off area: