Random improvements

Mainly fixing documentation, adhering to MediaWiki code conventions, etc.

Left a few @fixme's where I saw things that didn't look right but wasn't sure on how to fix them.
I'm glad you found this.  It should be self::$articleDatetime as set earlier by the onArticleViewHeader hook.  I'm embarrassed this made it into the released version.  Good thing I marked the TimeNegotiationForThumbnails option as EXPERIMENTAL.