[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 <yhuai@databricks.com>

Closes #10211 from yhuai/basePathDoc.

docs/sql-programming-guide.md

@@ -1233,6 +1233,13 @@ infer the data types of the partitioning columns. For these use cases, the autom
 can be configured by `spark.sql.sources.partitionColumnTypeInference.enabled`, which is default to
 `true`. When type inference is disabled, string type will be used for the partitioning columns.
 
+Starting from Spark 1.6.0, partition discovery only finds partitions under the given paths
+by default. For the above example, if users pass `path/to/table/gender=male` to either 
+`SQLContext.read.parquet` or `SQLContext.read.load`, `gender` will not be considered as a
+partitioning column. If users need to specify the base path that partition discovery
+should start with, they can set `basePath` in the data source options. For example,
+when `path/to/table/gender=male` is the path of the data and
+users set `basePath` to `path/to/table/`, `gender` will be a partitioning column.
 
 ### Schema Merging