Replace @returns with @return, in /lib
Fix @return array PHPDocs, in /lib
Squash 'a | b' into 'a|b', in /lib
Fix all PHPDoc types and variable names, in /lib
Change parameter type for useBackend
Fix PHPDoc and typos

Some things my IDE complained about while creating a custom backend.

Fix PHPDoc and typos

Some things my IDE complained about while creating a custom backend.

Conflicts:
	lib/private/group/database.php
	lib/private/group/dummy.php
	lib/private/group/example.php
Use OC_User_Interface instead of OC_User_Backend
Fix Scrutinizer errors about missing user backend