[SPARK-5183][SQL] Update SQL Docs with JDBC and Migration Guide

Author: Michael Armbrust <[email protected]>

Closes apache#4958 from marmbrus/sqlDocs and squashes the following commits:

9351dbc [Michael Armbrust] fix parquet example
6877e13 [Michael Armbrust] add sql examples
d81b7e7 [Michael Armbrust] rxins comments
e393528 [Michael Armbrust] fix order
19c2735 [Michael Armbrust] more on data source load/store
00d5914 [Michael Armbrust] Update SQL Docs with JDBC and Migration Guide

Author: marmbrus