post-5.18 release improvements

this is mostly doc, as usual

BUT there is also one commit which is about test suite. It is very similar to 6a469ee4 if you are wondering.

to clarify "More checks where I do errors", this is what happened:

• I ran ./publish-test-udf
• this failed because of irrelevant git errors:

  fatal: bad object refs/remotes/local/19237-test-suite-keyboard-input+force-all-tests
   error: gitlab-ssh.tails.boum.org:tails/tails.git did not send all necessary objects

  it's a kind of errors that I don't fully understand, and relates to how I also use "local" repositories so that I avoid unneedingly fetch from the network what I already have on my computer. I know how to fix it when I see it, but I wasn't using ${MASTER_CHECKOUT} so I missed it.
• I haven't noticed the errors :/
  • As a side note, this might be related to how too many of our commands return a non-zero exit code. A typical source of this issue is ending with a git commit even if we're not sure there is something to commit. I tried having loud warnings of errors, but now I'm into warning fatigue and don't notice that anymore.