docs: stable-kernel-rules: fine-tune various details

docs: stable-kernel-rules: fine-tune various details

* various fine tuning to the text that cleans up rough edges the three
  previous preparatory patches left behind to keep the diffs simpler
* s/Linus' tree/mainline/g, as that's the term more commonly used and
  known
* create a short intro for the three submission options and streamline
  the explanation when to use which of them
* fix a >= vs <= thinko in an example to make it more straight forward
* there were two blank lines before some sub-headings and just one
  before others; use the former style everywhere

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/e1960a70acae2c2f18b838aee9f8bf6055fae89b.1691219455.git.linux@leemhuis.info
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>