DefaultSettings: Add @since annotations.

Based on information from:
- https://www.mediawiki.org/wiki/Manual:$wgConf
- https://www.mediawiki.org/wiki/Manual:$wgVersion
- https://www.mediawiki.org/wiki/Manual:$wgCanonicalServer
- https://www.mediawiki.org/wiki/Manual:$wgUsePathInfo
- etc.

Change-Id: If359c57493de8efa3beb92b75e2ece9ca4a3a828