docs: document load_all_downstream() for cross-schema diagrams

Add usage examples to Diagram class docstring and
Dependencies.load_all_downstream() showing how to include
tables from unactivated downstream schemas in visualization.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: dimitri-yatsenko

src/datajoint/dependencies.py

@@ -235,6 +235,13 @@ def load_all_downstream(self) -> None:
         found. This ensures that cascade delete and drop reach all
         dependent tables, even those in schemas that haven't been
         explicitly activated.
+
+        Called automatically by ``Diagram.cascade()`` and ``Table.drop()``.
+        Call manually before constructing a ``Diagram`` to include
+        cross-schema dependencies in visualization::
+
+            conn.dependencies.load_all_downstream()
+            dj.Diagram(schema)  # now includes all downstream schemas
         """
         adapter = self._conn.adapter
         known_schemas = set(self._conn.schemas)

src/datajoint/diagram.py

@@ -80,7 +80,14 @@ class Diagram(nx.DiGraph):  # noqa: C901
     Notes
     -----
     ``diagram + 1 - 1`` may differ from ``diagram - 1 + 1``.
-    Only tables loaded in the connection are displayed.
+    Only tables in activated schemas are displayed. To include tables in
+    downstream schemas that depend on the current schema but haven't been
+    explicitly activated::
+
+        conn.dependencies.load_all_downstream()
+        dj.Diagram(schema)  # now includes all downstream schemas
+
+    ``Diagram.cascade()`` calls ``load_all_downstream()`` automatically.
 
     Layout direction is controlled via ``dj.config.display.diagram_direction``
     (default ``"TB"``). Use ``dj.config.override()`` to change temporarily::