$modeName: the requested content model name
&$handler: set this to a ContentHandler object, if desired.
+'ContentModelCanBeUsedOn': Called to determine whether that content model can
+be used on a given page. This is especially useful to prevent some content models
+to be used in some special location.
+$contentModel: ID of the content model in question
+$title: the Title in question.
+&$ok: Output parameter, whether it is OK to use $contentModel on $title.
+Handler functions that modify $ok should generally return false to prevent further
+hooks from further modifying $ok.
+
'ConvertContent': Called by AbstractContent::convert when a conversion to another
content model is requested.
$content: The Content object to be converted.