Follows-up 39a6e3dc4d (I8be497c623c5d92).
* Improve documentation all around and advertise 'class'
everywhere instead of 'uri'.
* Add test coverage for RCFeed::factory().
* Deprecate the $wgRCEngines "uri to class" mapping in favour
of specifying "class" directly in $wgRCFeeds.
* Deprecate RCFeedEngine in favour of FormattedRCFeed.
Convert to class_alias so that UDPRCFeedEngine no longer has
to extend the deprecated class name explicitly (for instanceof compat).
* Hard-deprecate RecentChange::getEngine.
Bug: T250628
Depends-On: Ie939e1d06b9ee2d841ec7256c8d24cc4e7e386dd
Change-Id: Ib6758d724c7200404c89c7ab157aa55f1cad9763
f8ecea1e5c