Merge pull request #431 from ankurdave/graphx-caching-doc

Describe caching and uncaching in GraphX programming guide

(cherry picked from commit ad294db326f57beb98f9734e2b4c45d9da1a4c89)
Signed-off-by: Reynold Xin <rxin@apache.org>
Merge branch 'branch-0.9' of https://git-wip-us.apache.org/repos/asf/incubator-spark into branch-0.9
Merge pull request #424 from jegonzal/GraphXProgrammingGuide

Additional edits for clarity in the graphx programming guide.

Added an overview of the Graph and GraphOps functions and fixed numerous typos.

(cherry picked from commit 3a386e238984c48a6ac07974b92647beae1199b3)
Signed-off-by: Reynold Xin <rxin@apache.org>