mediawiki.util: Clean up mw.util.$content

* Document how to use it (including the tricky bit with it needing
 a dependency on mediawiki.page.startup).

* Refer to mw.hook and #mw-content-text as recommended method of
  getting just the wikipage content.

* Remove the deprecated backwards-compatibility selectors.
  The core skins (Vector, Monobook, Modern, CologneBlue) all
  have mw-body now. Third-party skins should adopt those if they
  haven't already.

Change-Id: I30bdd893e0a0a340076efa7ce32fbd4b775bc3db
Merge "mediawiki.util: Clean up mw.util.$content"