SPARK-2148 Add link to requirements for custom equals() and hashcode() methods

https://issues.apache.org/jira/browse/SPARK-2148

Author: Andrew Ash <andrew@andrewash.com>

Closes #1092 from ash211/SPARK-2148 and squashes the following commits:

93513df [Andrew Ash] SPARK-2148 Add link to requirements for custom equals() and hashcode() methods
(cherry picked from commit 9672ee07fb1c3583c70f23a699de3b2282eb0f98)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>