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.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>