Fix the issues reported by phpcs

Mixed is a valid type in PHP 8 as per https://php.watch/versions/8.0/mixed-type and in its watchdog_watchdog.info.yml this module declares it is PHP 8 dependent, so I think we're good there.

Also, I'm not sure about keeping markdown lines to 80 characters. According to https://stackoverflow.com/questions/29955688/in-markdown-is-there-an-off..., "There is no best practice for wrapping in Markdown. The only mention of wrapping in the original spec says that hard wrapping is permitted:", not _required_.

Unless we have a compelling reason to wrap to 80 lines, I prefer to keep it as is.