docs: stable-kernel-rules: explain use of stable@kernel.org (w/o @vger.)

Document when to use of stable@kernel.org instead of
stable@vger.kernel.org, as the two are easily mixed up and their
difference not explained anywhere[1].

Link: https://lore.kernel.org/all/20240422231550.3cf5f723@sal.lan/ [1]
Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/6783b71da48aac5290756343f58591dc42da87bc.1714367921.git.linux@leemhuis.info

Documentation/process/stable-kernel-rules.rst

@@ -72,6 +72,10 @@ for stable trees, add this tag in the sign-off area::
 
   Cc: stable@vger.kernel.org
 
+Use ``Cc: stable@kernel.org`` instead when fixing unpublished vulnerabilities:
+it reduces the chance of accidentally exposing the fix to the public by way of
+'git send-email', as mails sent to that address are not delivered anywhere.
+
 Once the patch is mainlined it will be applied to the stable tree without
 anything else needing to be done by the author or subsystem maintainer.