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

Documentation/process/stable-kernel-rules.rst

@@ -6,7 +6,7 @@ Everything you ever wanted to know about Linux -stable releases
 Rules on what kind of patches are accepted, and which ones are not, into the
 "-stable" tree:
 
- - It or an equivalent fix must already exist in Linus' tree (upstream).
+ - It or an equivalent fix must already exist in Linux mainline (upstream).
  - It must be obviously correct and tested.
  - It cannot be bigger than 100 lines, with context.
  - It must follow the
@@ -134,7 +134,7 @@ comment to pass arbitrary or predefined notes:
 
    .. code-block:: none
 
-     Cc: <stable@vger.kernel.org> # after 4 weeks in mainline
+     Cc: <stable@vger.kernel.org> # after -rc3
 
  * Point out known problems: