[SPARK-4916][SQL][DOCS]Update SQL programming guide about cache section

`SchemeRDD.cache()` now uses in-memory columnar storage.

Author: luogankun <luogankun@gmail.com>

Closes #3759 from luogankun/SPARK-4916 and squashes the following commits:

7b39864 [luogankun] [SPARK-4916]Update SQL programming guide
6018122 [luogankun] Merge branch 'master' of https://github.com/apache/spark into SPARK-4916
0b93785 [luogankun] [SPARK-4916]Update SQL programming guide
99b2336 [luogankun] [SPARK-4916]Update SQL programming guide