docs: stable-kernel-rules: create special tag to flag 'no backporting'

Document a new variant of the stable tag developers can use to make the
stable team's tools ignore a change[1].

That way developers can use 'Fixes:' tags without fearing the changes
might be backported in semi-automatic fashion. Such concerns are the
reason why some developers deliberately omit the 'Fixes:' tag in
changes[2] -- which somewhat undermines the reason for the existence of
that tag and might be unwise in the long term[3].

Link: https://lore.kernel.org/all/b452fd54-fdc6-47e4-8c26-6627f6b7eff3@leemhuis.info/ [1]
Link: https://lore.kernel.org/all/cover.1712226175.git.antony.antony@secunet.com/ [2]
Link: https://lore.kernel.org/all/dfd87673-c581-4b4b-b37a-1cf5c817240d@leemhuis.info/ [3]
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/35989d3b2f3f8cf23828b0c84fde9b17a74be97c.1714367921.git.linux@leemhuis.info