* siri/supervisor/warn-shutdown-race/ERL-724:
  [supervisor] Add warning about race condition

ERL-724: "During a 'gentle' shutdown, supervisors unlink from their
children before sending shutdown signals to them. This can lead to a
race condition in supervision trees, when the timeout for gentle
shutdown of a parent supervisor expires and it kills a child
supervisor that has just unlinked from a child of its own, leaving the
child supervisor's own child still running after its supervisor is
killed."

This commit adds a warning about this in the documentation.

* rickard/pcre-8.42/OTP-15217:
  Update PCRE from version 8.41 to version 8.42

Document allowed integer ops in types

Refer to the OTP Versions Tree in the documentation in order to
better illustrate how the version scheme works

In the documentation, we don't use contractations such as
"don't" or "that's". We also avoid abbreviations. "i.e."
should be written out as "that is". In this case it is
better to omit it altogether.

629ad2e0971ef introduced the violation.

As explained by @psyeugenic on Slack:
> empty list `[]` is regarded as a separate type from `list()`, `[]` < `list()`
> i.e. nil is `[]`