Document db schema escape hatch

We had an instance where a user dropped metadata tables. Using schema migrations recovery from this is not crystal-clear as the state of the database doesn't map to any of the known versions.

You can generally fix this with `Base.metadata.create_all`, but most people aren't just going to know this. We should add a section in the documentation.

Start by locating the documentation section covering schema migrations and review the recovery context described in the issue. Document when migration state no longer matches the database and explain the Base.metadata.create_all recovery path; done means a reader can follow the recovery steps without prior knowledge.

Written by the indexing model from the issue text.