Minor revisions to dx-{download,upload}-all-* help messages.

Summary: Also format the help strings a bit more nicely in the source code.

Test Plan: Manually examined output of running both scripts with -h.

Reviewers: orodeh, klai

Reviewed By: klai

Differential Revision: https://phabricator.dnanexus.com/D31

(cherry picked from commit 3eea10495170faa26545b8d9c7ee661e053fdf75)