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