Documentation for compound foreign key support, refs #594

- Changelog entries for all the compound foreign key work
- Upgrading guide notes the transform() behavior changes
- ForeignKey added to the API reference
- Updated .foreign_keys introspection example output

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/reference.rst

@@ -70,6 +70,13 @@ sqlite_utils.db.ColumnDetails
 
 .. autoclass:: sqlite_utils.db.ColumnDetails
 
+.. _reference_db_other_foreign_key:
+
+sqlite_utils.db.ForeignKey
+--------------------------
+
+.. autoclass:: sqlite_utils.db.ForeignKey
+
 sqlite_utils.utils
 ==================