* mattermost: add support for message priority
* Add changelog fragment
* Consistency nit in changelog
Co-authored-by: Alexei Znamensky <103110+russoz@users.noreply.github.com>
* Validate priority arg and doc fixes
Validate the two possible priorities with choices.
Add priority arg to one example and add version_added
field for the arg docs.
* Update changelog.
---------
Co-authored-by: Alexei Znamensky <103110+russoz@users.noreply.github.com>
Co-authored-by: Felix Fontein <felix@fontein.de>
