docs: stable-kernel-rules: mention that regressions must be prevented

Document that changes intended for a specific stable series have to be
in all newer series still maintained, as otherwise users would run into
regressions.

CC: Greg KH <gregkh@linuxfoundation.org>
CC: Sasha Levin <sashal@kernel.org>
CC: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
Link: https://lore.kernel.org/r/ddb5cb0d6b7aa4ef31642cd9657a0fb53d79cddb.1691219455.git.linux@leemhuis.info


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>