Got rid of some global $wgContLang usage in SpecialAllpages

Using $this->getContext()->getLanguage() instead now. In SpecialAllpages::showline
the global is still being used and a comment added. It could be argued about whether
the content language or the user language should be used in that place.

Also added some missing documentation to Language::getNamespaces.

Change-Id: If511e2d9edb0f0092f3ca0ceadabd7fdb72f6c06