- Various Fixes to docs
 - Make data source traits actually interfaces

Based on #4862 but with fixed conflicts.

Author: Reynold Xin <rxin@databricks.com>
Author: Michael Armbrust <michael@databricks.com>

Closes #4868 from marmbrus/pr/4862 and squashes the following commits:

fe091ea [Michael Armbrust] Merge remote-tracking branch 'origin/master' into pr/4862
0208497 [Reynold Xin] Test fixes.
34e0a28 [Reynold Xin] [SPARK-5310][SQL] Various fixes to Spark SQL docs.