Update digitization_manifest.py help text

[HaSistrunk]

Edit arguments, options, and help text to better clarify how to run the script.

[helrond]

The text that's shown in the comments at the top of the file should be identical to what is generated when calling the help command of the script (./digitization_manifest.py -h) so let's make sure that the arguments defined by argparse are correct first. I'm still seeing a mismatch there.