Update documentation for add_adb_serial and construct_adb_shell_command functions in util.py.
Update documentation for add_adb_serial and construct_adb_shell_command functions in util.py.

Review-Url: https://codereview.chromium.org/1777983002

sahiljain/catapult,sahiljain/catapult,benschmaus/catapult,catapult-project/catapult-csm,benschmaus/catapult,benschmaus/catapult,catapult-project/catapult-csm,catapult-project/catapult,catapult-project/catapult,sahiljain/catapult,catapult-project/catapult-csm,catapult-project/catapult,benschmaus/catapult,catapult-project/catapult-csm,catapult-project/catapult,catapult-project/catapult,benschmaus/catapult,catapult-project/catapult-csm,catapult-project/catapult-csm,sahiljain/catapult,benschmaus/catapult,catapult-project/catapult,sahiljain/catapult,catapult-project/catapult,benschmaus/catapult,catapult-project/catapult-csm,sahiljain/catapult