Updates docs for ScalaPB Validation

[fedefernandez]

What does this change do?

Updates the docs to document the changes implemented at https://github.com/higherkindness/sbt-mu-srcgen/pull/304

It depends on that PR. The steps will be:

• [x] Get approval on both PRs (get the feature approved)
• [x] Merge the plugin's PR and test it
• [ ] Merge this and release mu-scala
• [ ] Update the plugin with the new mu-scala version
• [ ] Release the plugin

Checklist

• [x] Reviewed the diff to look for typos, println and format errors.
• [x] Updated the docs accordingly.