Add various doc updates

They are still very far from complete.

docs/migrations-and-seeders-and-database-table-ownership.md

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+Some database tables are owned by diachron and some are owned by the
+application.
+
+This also applies to seeders: some are owned by diachron and some by the
+application.
+
+The database's structure is managed by migrations written in SQL.
+
+Each migration gets its own file.  These files' names should match
+`yyyy-mm-dd_ss-description.sql`, eg `2026-01-01_01-users.sql`.
+
+Files are sorted lexicographically by name and applied in order.
+
+Note: in the future we may relax or modify the restriction on migration file
+names, but they'll continue to be applied in lexicographical order.
+
+## framework and application migrations
+
+Migrations owned by the framework are kept in a separate directory from those
+owned by applications.  Pending framework migrations, if any, are applied
+before pending application migrations, if any.
+
+diachron will go to some lengths to ensure that framework migrations do not
+break applications.
+
+## no downward migrations
+
+diachron does not provide them.  "The only way out is through."
+
+When developing locally, you can use the command `develop reset-db`.  **NEVER
+USE THIS IN PRODUCTION!**  Always be sure that you can "get back to where you
+were".  Being careful when creating migrations and seeders can help, but
+dumping and restoring known-good copies of the database can also take you a
+long way.