Improve parallel_run.py somewhat

Make parallel_run just wait for any remaining jobs to finish before
exiting. Attempting to exit early generally didn't work very well.

Also improve documentation a bit.

Test: ./art/tools/parallel_run.py -j80 runit.sh
Change-Id: Ie8b3f1b4ad9bbc03a04e6bdc8e8cea6140b85f7f
1 file changed