Remove ROOT_DIR everywhere it is mentioned and replace it with build root.

Testing Done:
$ ack ROOT_DIR
(no results)

(since this is only docstring changes, no need for tests)

Reviewed at https://rbcommons.com/s/twitter/r/1084/
Sync of https://github.com/pantsbuild/pants/commit/a503f0858b85bf50884ac0587fccf1467587fa61