[Enhancement] Improve documentation for restore

[pboushy]

Summary

I noticed there is a test in the source code for a RestoreJobManager that looks like it can:

1. Determine total size of data to download.
2. Kickoff restore.
3. Check the status of the restore on a polling interval.

Please document how we can do 1, 2, and 3 using the py42 library.
What the test code is doing looks to be very similar to what I'm attempting, but it looks like mostly private APIs or something unique that can't really be done with sdk.archive right now.