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>