This PR adds document for `basePath`, which is a new parameter used by `HadoopFsRelation`.

The compiled doc is shown below.
![image](https://cloud.githubusercontent.com/assets/2072857/11673132/1ba01192-9dcb-11e5-98d9-ac0b4e92e98c.png)

JIRA: https://issues.apache.org/jira/browse/SPARK-11678

Author: Yin Huai <yhuai@databricks.com>

Closes #10211 from yhuai/basePathDoc.