Update help text for `pmtiles extract`

[rmx90210]

Thank you for PMTiles — it's been so helpful to me!

While reading the docs for pmtiles extract, I noticed a few things that could be tweaked for clarity:

1. --bbox isn't listed under Options
2. --region says "a GeoJSON Polygon, Multipolygon, Feature, or FeatureCollection", but I think it might help to add the purpose of the option and how it will be used (i.e. is it a spatial join between the features and tiles?)
3. It would be helpful to know what the default are for any options that have them.

I think these minor edits will help newcomers like myself get started with the CLI even faster 😄

[bdon]

Can you please open a PR with your suggested changes?