[feature/auth-refactor] Document the provider interface

Provides basic documentation of the auth_provideR_interface.
Changes the login method to login($username, $password) for
consistency with the providers.
acp() is not fully documented. It appears that it is meant to
return an array of some sort and take in a variable by reference.

PHPBB3-9734
[feature/auth-refactor] Fix comment block indentation

Comment block indentation was off by one space on the provider_*
files due to being incorrectly copied over from the auth_* files.

PHPBB3-9734