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>

sqlite_utils/db.py

@@ -176,6 +176,9 @@ class ForeignKey:
     are ``None`` - use ``columns`` and ``other_columns`` instead, and check
     ``is_compound``.
 
+    ``on_delete`` and ``on_update`` hold the foreign key actions, e.g.
+    ``"CASCADE"`` - ``"NO ACTION"`` if not set.
+
     Prior to sqlite-utils 4.0 this was a ``namedtuple`` and could be unpacked
     or indexed as ``(table, column, other_table, other_column)``. It is now a
     dataclass - access its fields by name instead.