Docs: clarify when hooks stop after the first non-None result

[pkch]

Fixes #2493

• Add a new news fragment into the changelog folder
  • name it $issue_id.$type for example (588.bug)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of removal, feature, bugfix, vendor, doc or trivial
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."
• Target: for bugfix, vendor, doc or trivial fixes, target master; for removals or features target features;

[coveralls]

Coverage remained the same at 92.128% when pulling 4a992ba on pkch:firstresult into f0541b6 on pytest-dev:master.

[RonnyPfannschmidt]

Thanks 👍