[SPARK-7084] improve saveAsTable documentation

Author: madhukar <phatak.dev@gmail.com>

Closes #5654 from phatak-dev/master and squashes the following commits:

386f407 [madhukar] #5654 updated for all the methods
2c997c5 [madhukar] Merge branch 'master' of https://github.com/apache/spark
00bc819 [madhukar] Merge branch 'master' of https://github.com/apache/spark
2a802c6 [madhukar] #5654 updated the doc according to comments
866e8df [madhukar] [SPARK-7084] improve saveAsTable documentation