Improve documentation of the PageContentLanguage hook

Improve documentation of the PageContentLanguage hook

Clarify that the type of second parameter that is being passed
to the hooks can be anything because one hook can change it and
then it is passed to another hook without the hook caller having
possibility to check or modify the value.

Clarify that hooks should only return Language objects.

Rename $wgLang to $userLang in the hook parameter documentation to
avoid false posivite matches for the global.

Fix some typos, use Title::inNamespace and add a test assertion.

Also, the $content parameter is unused by all implementations of
this method, and on quick look never passed by any caller. I kept
it for now, however.

Bug: T214358
Change-Id: Iae49d2998c2b762565d232c0337d84d43a4a900c