I've added documentation clarifying the particular lack of clarity highlighted in the relevant JIRA. I've also added code examples for this issue to clarify the explanation.

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

Closes #4022 from ilganeli/SPARK-733 and squashes the following commits:

587def5 [Ilya Ganelin] Updated to clarify verbage
df3afd7 [Ilya Ganelin] Revert "Partially updated task metrics to make some vars private"
3f6c512 [Ilya Ganelin] Revert "Completed refactoring to make vars in TaskMetrics class private"
58034fb [Ilya Ganelin] Merge remote-tracking branch 'upstream/master' into SPARK-733
4dc2cdb [Ilya Ganelin] Merge remote-tracking branch 'upstream/master' into SPARK-733
3a38db1 [Ilya Ganelin] Verified documentation update by building via jekyll
33b5a2d [Ilya Ganelin] Added code examples for java and python
1fd59b2 [Ilya Ganelin] Updated documentation for accumulators to highlight lazy evaluation issue
5525c20 [Ilya Ganelin] Completed refactoring to make vars in TaskMetrics class private
c64da4f [Ilya Ganelin] Partially updated task metrics to make some vars private