polish documentation based on scrutinizer patches

@DeepDiver1975 pointed me to scrutinizer. I took the patches https://scrutinizer-ci.com/g/owncloud/core/inspections/7321afa0-d5b1-4d44-9bbc-1a9797bd872e/patches and removed the unused import cleanup ... also changed some of the documentation comments.

This PR cleans up a lot of missing or wrong documentation.