[SPARK-5750][SPARK-3441][SPARK-5836][CORE] Added documentation explaining shuffle

I've updated the Spark Programming Guide to add a section on the shuffle operation providing some background on what it does. I've also addressed some of its performance impacts.

I've included documentation to address the following issues:
https://issues.apache.org/jira/browse/SPARK-5836
https://issues.apache.org/jira/browse/SPARK-3441
https://issues.apache.org/jira/browse/SPARK-5750

https://issues.apache.org/jira/browse/SPARK-4227 is related but can be addressed in a separate PR since it involves updates to the Spark Configuration Guide.

Author: Ilya Ganelin <ilya.ganelin@capitalone.com>
Author: Ilya Ganelin <ilganeli@gmail.com>

Closes #5074 from ilganeli/SPARK-5750 and squashes the following commits:

6178e24 [Ilya Ganelin] Update programming-guide.md
7a0b96f [Ilya Ganelin] Update programming-guide.md
2c5df08 [Ilya Ganelin] Merge branch 'SPARK-5750' of github.com:ilganeli/spark into SPARK-5750
dffbd2d [Ilya Ganelin] [SPARK-5750] Slight wording update
1ff4eb4 [Ilya Ganelin] Merge remote-tracking branch 'upstream/master' into SPARK-5750
85f9c6e [Ilya Ganelin] Update programming-guide.md
349d1fa [Ilya Ganelin] Added cross linkf or configuration page
eeb5a7a [Ilya Ganelin] [SPARK-5750] Added some minor fixes
dd5cc9d [Ilya Ganelin] [SPARK-5750] Fixed some factual inaccuracies with regards to shuffle internals.
a8adb57 [Ilya Ganelin] [SPARK-5750] Incoporated feedback from Sean Owen
9954bbe [Ilya Ganelin] Merge remote-tracking branch 'upstream/master' into SPARK-5750
159dd1c [Ilya Ganelin] [SPARK-5750] Style fixes from rxin.
75ef67b [Ilya Ganelin] [SPARK-5750][SPARK-3441][SPARK-5836] Added documentation explaining the shuffle operation and included errata from a number of other JIRAs