Implement Postgresql `COMMENT` with documentation comments

Doc comments would be neat and pretty easy to add in via migrations.

Adding them is easy. COMMENT ON COLUMN ${table}.${column} IS '${comment text goes here}' for columns and COMMENT ON TABLE $(table) IS '$(comment text)'.

Checking to see if we need to add them, though, is tricky. We need to provide the col_description(table_oid, column_number). This looks like col_description('foo'::regclass, 0) to get the first column. I don't think we can assume that the table has the same column ordering as the Persistent representation, so we'd want to associate the column names with their indices.

This StackOverflow question has some helpful info for retrieving the comments in SQL.

The issue names no files, tests, or entry points. Start by tracing the PostgreSQL migration path and how the Persistent representation maps column names, then consult PostgreSQL COMMENT and col_description behavior; done means adding and checking table and column documentation comments without relying on column order.