[SPARK-4668] Fix some documentation typos.

Author: Ryan Williams <ryan.blake.williams@gmail.com>

Closes #3523 from ryan-williams/tweaks and squashes the following commits:

d2eddaa [Ryan Williams] code review feedback
ce27fc1 [Ryan Williams] CoGroupedRDD comment nit
c6cfad9 [Ryan Williams] remove unnecessary if statement
b74ea35 [Ryan Williams] comment fix
b0221f0 [Ryan Williams] fix a gendered pronoun
c71ffed [Ryan Williams] use names on a few boolean parameters
89954aa [Ryan Williams] clarify some comments in {Security,Shuffle}Manager
e465dac [Ryan Williams] Saved building-spark.md with Dillinger.io
83e8358 [Ryan Williams] fix pom.xml typo
dc4662b [Ryan Williams] typo fixes in tuning.md, configuration.md

(cherry picked from commit 8176b7a02e6b62bbce194c3ce9802d58b7472101)
Signed-off-by: Patrick Wendell <pwendell@gmail.com>

Conflicts:
	pom.xml