docs: verify/bisect: drop 'v' prefix, EOL aspect, and assorted fixes

A bunch of minor fixes and improvements and two other things:

- Explain the 'v' version prefix when it's first used, but drop it
  everywhere in the text for consistency. Also drop single quotes around
  a few version numbers.

- Point out that testing a stable/longterm kernel only makes sense if
  the series is still supported.
Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <f13d203d5975419608996300992eaa2e4fcc2dc1.1710750972.git.linux@leemhuis.info>