[SPARK-6800][SQL] Update doc for JDBCRelation's columnPartition

JIRA https://issues.apache.org/jira/browse/SPARK-6800

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #5488 from viirya/fix_jdbc_where and squashes the following commits:

51386c8 [Liang-Chi Hsieh] Update code comment.
1dcc929 [Liang-Chi Hsieh] Update document.
3eb74d6 [Liang-Chi Hsieh] Revert and modify doc.
df11783 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into fix_jdbc_where
3e7db15 [Liang-Chi Hsieh] Fix wrong logic to generate WHERE clause for JDBC.