docs: handling-regressions: rework section about fixing procedures

This basically rewrites the 'Prioritize work on fixing regressions'
section of Documentation/process/handling-regressions.rst for various
reasons. Among them: some things were too demanding, some didn't align
well with the usual workflows, and some apparently were not clear enough
-- and of course a few things were missing that would be good to have in
there.

Linus for example recently stated that regressions introduced during the
past year should be handled similarly to regressions from the current
cycle, if it's a clear fix with no semantic subtlety. His exact
wording[1] didn't fit well into the text structure, but the author tried
to stick close to the apparent intention.

It was a noble goal from the original author to state "[prevent
situations that might force users to] continue running an outdated and
thus potentially insecure kernel version for more than two weeks after a
regression's culprit was identified"; this directly led to the goal "fix
regression in mainline within one week, if the issue made it into a
stable/longterm kernel", because the stable team needs time to pick up
and prepare a new release. But apparently all that was a bit too
demanding.

That "one week" target for example doesn't align well with the usual
habits of the subsystem maintainers, which normally send their fixes to
Linus once a week; and it doesn't align too well with stable/longterm
releases either, which often enter a -rc phase on Mondays or Tuesdays
and then are released two to three days later. And asking developers to
create, review, and mainline fixes within one week might be too much to
ask for in general. Hence tone the general goal down to three weeks and
use an approach that better aligns with the usual merging and release
habits.

While at it, also make the rules of thumb a bit easier to follow by
grouping them by topic (e.g. generic things, timing, procedures, ...).

Also add text for a few cases where recent discussions showed they need
covering. Among them are multiple points that better explain the
relations to stable and longterm kernels and the team that manages them;
they and the group seperators are the primary reason why this whole
section sadly grew somewhat in the rewrite.

The group about those relations led to one addition the author came up
with without any precedent from Linus: the text now tells developers to
add a stable tag for any regression that made it into a proper mainline
release during the past 12 months. This is meant to ensure the stable
team will definitely notice any fixes for recent regressions. That
includes those introduced shortly before a new mainline release and
found right after it; without such a rule the stable team might miss the
fix, which then would only reach users after weeks or months with later
releases.

Note, the aspect "Do not consider regressions from the current cycle as
something that can wait till the cycle's end [...]" might look like an
addition, but was kinda was in the old text as well -- but only
indirectly. That apparently was too subtle, as many developers seem to
assume waiting till the end of the cycle is fine (even for build
fixes).

In practice this was especially problematic when a cause of a regression
made it into a proper release (either directly or through a backport). A
revert performed by Linus shortly before the 6.3 release illustrated
that[2], as the developer of the culprit had been willing to revert the
culprit about three weeks earlier already -- but didn't do so when a fix
came into sight and a maintainer suggested it can wait. Due to that the
issue in the end plagued users of 6.2.y at least two weeks longer than
necessary, as the fix in the end didn't become ready in time. This issue
in fact could have been resolved one or two additional weeks earlier, if
the developer had reverted the culprit shortly after it had been
identified (which even the old version of the text suggest to do in such
cases).

[1] https://lore.kernel.org/all/CAHk-=wis_qQy4oDNynNKi5b7Qhosmxtoj1jxo5wmB6SRUwQUBQ@mail.gmail.com/

[2] https://lore.kernel.org/all/CAHk-=wgD98pmSK3ZyHk_d9kZ2bhgN6DuNZMAJaV0WTtbkf=RDw@mail.gmail.com/

CC: Linus Torvalds <torvalds@linux-foundation.org>
CC: Greg KH <gregkh@linuxfoundation.org>
CC: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/6971680941a5b7b9cb0c2839c75b5cc4ddb2d162.1684139586.git.linux@leemhuis.info
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

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