[SPARK-11678][SQL][DOCS] Document basePath in the programming guide.
[SPARK-11678][SQL][DOCS] Document basePath in the programming guide.

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 <8adc4758e0403b3bd961e4e414291eaa9ba64098@databricks.com>

Closes #10211 from yhuai/basePathDoc.

(cherry picked from commit ac8cdf1cdc148bd21290ecf4d4f9874f8c87cc14)
Signed-off-by: Yin Huai <8adc4758e0403b3bd961e4e414291eaa9ba64098@databricks.com>

haowu80s/spark,haowu80s/spark,haowu80s/spark,haowu80s/spark,haowu80s/spark,haowu80s/spark