What does this change

Include an explanation on the website and in the command doc on how the storage migrate command behaves. Warn people to not repeat the migration after they have used Porter v1 with the migrated database.

What issue does it fix

I realized that the migration behavior wasn't documented when I was helping a user with their migration.

Notes for the reviewer

N/A

Checklist

• Did you write tests?
• Did you write documentation?
• Did you change porter.yaml or a storage document record? Update the corresponding schema file.
• If this is your first pull request, please add your name to the bottom of our Contributors list. Thank you for making Porter better! 🙇‍♀️

Reviewer Checklist

• Comment with /azp run test-porter-release if a magefile or build script was modified
• Comment with /azp run porter-integration if it's a non-trivial PR