Improve in-line documentation of $wgUrlProtocols; add 'redis://'

* RedisPubSubFeedEngine uses 'redis://' URLs. So does redis-py and many other
  client libraries. It should be recognized by MediaWiki. So add it.
* Remove in-line comments justifying each individual URL scheme.
  DefaultSettings.php is long enough; it doesn't need to moonlight as
  the annals of the history of MediaWiki development.
* Make the documentation of $wgUrlProtocols a bit more useful.

Change-Id: I4c2de67cc4e243659b0bed674e0ccd48221c57f3
Merge "Improve in-line documentation of $wgUrlProtocols; add 'redis://'"