I've attached to this mail two improvements for the documentation groups
feature that is new in GNU Emacs 28:

- Fixed a typo in the manual and improved a bit some explanations and
  docstrings that were not very clear to me when I read them.

- Added a new documentation group that describes what you can do with
  buffer overlays in Emacs (should we have a similar one for text
  properties?).

Thanks, I hope you find them useful.