Reorder and update metadata docstring, convert last_update field to datetime

[chinandrew]

Closes #109

Summary of changes:

• Reorder values in docstring to align with actual returned values
• Add documentation for columns that weren't previously included.

TODO:

• convert timestamp for last updated to datetime instead of unix time.

┆Issue is synchronized with this Asana task by Unito

[capnrefsmmat]

There's an incoming new column that may get added soon: cmu-delphi/delphi-epidata#236

But otherwise I like this change.

[capnrefsmmat]

Merge conflict now, but I think this is good to merge when fixed (unless we want to wait for cmu-delphi/delphi-epidata#236)

[chinandrew]

Merge conflict now, but I think this is good to merge when fixed (unless we want to wait for cmu-delphi/delphi-epidata#236)

Thanks, conflict resolved. I don't have a good sense of timeline for when that linked pr will be merged, but figure it's good to keep things as correct as possible and change later if things change (even if we never actually publish new docs publicly).