$rc: RecentChange being tagged when the tagging accompanies the action or null
$user: User who performed the tagging when the tagging is subsequent to the action or null
+'ChangeUserGroups': Called before user groups are changed.
+$performer: The User who will perform the change
+$user: The User whose groups will be changed
+&$add: The groups that will be added
+&$remove: The groups that will be removed
+
'Collation::factory': Called if $wgCategoryCollation is an unknown collation.
$collationName: Name of the collation in question
&$collationObject: Null. Replace with a subclass of the Collation class that
test case files matching the suffix "Test.php".
&$paths: list of test cases and directories to search.
-'UnknownAction': An unknown "action" has occurred (useful for defining your own
-actions).
+'UnknownAction': DEPRECATED! To add an action in an extension,
+create a subclass of Action, and add a new key to $wgActions.
+An unknown "action" has occurred (useful for defining your own actions).
$action: action name
$article: article "acted on"