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>
Updating docs to include missing information about reducers and clarify ...

...how the OFFHEAP storage level works (there has been confusion around this).

Author: Ali Ghodsi <alig@cs.berkeley.edu>

Closes #1089 from alig/master and squashes the following commits:

ca8114d [Ali Ghodsi] Updating docs to include missing information about reducers and clarify how the OFFHEAP storage level works (there has been confusion around this).
(cherry picked from commit 119b06a04f6df3949b3b074a18f791bbc732ac31)

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