docs: stable-kernel-rules: call mainline by its name and change example

Fine-tuning:

* s/Linus' tree/Linux mainline/, as mainline is the term used elsewhere
  in the document.

* Provide a better example for the 'delayed backporting' case that uses
  a fixed rather than a relative reference point, which makes it easier
  to handle for the stable team.

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/0a120573ea827aee12d45e7bd802ba85c09884da.1714367921.git.linux@leemhuis.info