[SPARK-4658][SQL] Code documentation issue in DDL of datasource API

Author: ravipesala <ravindra.pesala@huawei.com>

Closes #3516 from ravipesala/ddl_doc and squashes the following commits:

d101fdf [ravipesala] Style issues fixed
d2238cd [ravipesala] Corrected documentation

(cherry picked from commit bc353819cc86c3b0ad75caf81b47744bfc2aeeb3)
Signed-off-by: Michael Armbrust <michael@databricks.com>