docs: stable-kernel-rules: reduce redundancy

Explain the general concept once in the intro to keep things somewhat
shorter in the individual points.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/106e21789e2bf02d174e1715b49cd4d30886d51f.1714367921.git.linux@leemhuis.info