docs(migration): address SQLAlchemy review follow-ups

Four operator-experience improvements from the second review pass:

1. ``TABLES_WITH_NULLABLE_FKS`` is now explicitly documented as an
   informational set that is not consulted at runtime; the comment
   explains the previous ``dashboard_roles`` omission was the bug
   that motivated the always-run cleanup.
2. ``_delete_null_fk_rows`` docstring updated to match the
   "always run" semantics (was still claiming "called only on tables
   in TABLES_WITH_NULLABLE_FKS").
3. ``_check_no_external_fks_to_id`` now documents its scope
   limitation: ``Inspector.get_table_names()`` returns the default
   schema only, so cross-schema FKs in non-standard multi-schema
   PostgreSQL deployments would not be caught. The single-schema
   case (Superset's documented deployment) is fully covered.
4. ``_dedupe_by_min_id`` now logs a sample of up to 10 discarded
   ``(fk1, fk2, id)`` tuples at WARN before deletion, so operators
   can audit which rows the ``MIN(id)`` policy drops. The keep-
   original policy is correct in practice but discards later
   re-grants on ownership tables; the sample makes that visible.
5. ``UPDATING.md`` documents the upgrade/downgrade primary-key
   name divergence (``pk_<table>`` vs ``<table>_pkey``) so
   operators using schema-comparison tools don't mistake it for
   migration drift.

No schema or runtime-behaviour changes. All 44 migration tests pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>