docs: stable-kernel-rules: make rule section more straight forward

docs: stable-kernel-rules: make rule section more straight forward

Tweak some of the rule text to make things more straight forward, with
the goal to stick closely to the intend of the old text:

* put the "it or equivalent fix must be upstream" rule at the top, as
  it's one of the most important ones that at the same time often seems
  to be missed by developers.
* "It must fix only one thing" was dropped, as that is almost always a
  thing that needs to be handled earlier when the change is mainlined.
  Furthermore, this is already indirectly covered by the "Separate your
  changes" section in Documentation/process/submitting-patches.rst which
  the rules already point to.
* six other rules are in the end one rule with clarifications; structure
  the text accordingly to make it a lot easier to follow and understand
  the intend.
* drop the 'In short, something critical' from one of those notes: it
  contradicts the "real bug that bothers people" aspect somewhat and does
  not really add anything

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/f83e812879caa978a51a1a7cae7c359f29fc093c.1689056247.git.linux@leemhuis.info
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>