(maint) Add documentation on side effects in tests
(#18895) Fix documentation in runit provider

The documentation and error messages for the runit provider made a couple of
statements that were inconsistent with the code:

  - `/etc` is not checked as a daemon directory. The path checked is `/etc/sv`.

  - `/var/lib/service` is checked as a daemon directory, but this wasn't
    documented.

  - The service directory `/etc/service` is checked before `/var/service`.
Merge pull request #1639 from Sharpie/18895-fix-runit-documentation

(#18895) Fix documentation in runit provider
Merge pull request #1636 from adrienthebo/maint-readme_developer_explain_side_effects_in_tests

(maint) Add documentation on side effects in tests
Merge branch 'master' of https://github.com/puppetlabs/puppet