Author: Davies Liu <davies@databricks.com>

Closes #6311 from davies/rollup and squashes the following commits:

0261db1 [Davies Liu] use @since
a51ca6b [Davies Liu] Merge branch 'master' of github.com:apache/spark into rollup
8ad5af4 [Davies Liu] Update dataframe.py
ade3841 [Davies Liu] add DataFrame.rollup/cube in Python