Document new RegionProcedureStore operation and migration

Add a few notes to the refguide on the new RegionProcedureStore, how it works (A 'Region' but buried in the Master with dedicated flushing/compacting threads and archivers for WAL and hfile), how it differs from WALPS, and note it auto-migrates and there should be new issue moving on to the new store.

Mention the configuration. Mention it is on WALFS even though it is a 'Region', etc.