[SPARK-4652][DOCS] Add docs about spark-git-repo option

There might be some cases when WIPS spark version need to be run
on EC2 cluster. In order to setup this type of cluster more easily,
add --spark-git-repo option description to ec2 documentation.

Author: lewuathe <lewuathe@me.com>
Author: Josh Rosen <joshrosen@databricks.com>

Closes #3513 from Lewuathe/doc-for-development-spark-cluster and squashes the following commits:

6dae8ee [lewuathe] Wrap consistent with other descriptions
cfaf9be [lewuathe] Add docs about spark-git-repo option

(Editing / cleanup by Josh Rosen)