Documentation: added bpy.msgbus description and examples

The `bpy.msgbus` namespace was not included in the documentation generation.
I've added it, and ported Campbell's examples from P563.

doc/python_api/examples/bpy.msgbus.1.py

@@ -0,0 +1,44 @@
+"""
+The message bus system can be used to receive notifications when properties of
+Blender datablocks are changed via the data API.
+
+
+Limitations
+-----------
+
+The message bus system is triggered by updates via the RNA system. This means
+that the following updates will result in a notification on the message bus:
+
+- Changes via the Python API, for example ``some_object.location.x += 3``.
+- Changes via the sliders, fields, and buttons in the user interface.
+
+The following updates do **not** trigger message bus notifications:
+
+- Moving objects in the 3D Viewport.
+- Changes performed by the animation system.
+
+
+Example Use
+-----------
+
+Below is an example of subscription to changes in the active object's location.
+"""
+
+import bpy
+
+# Any Python object can act as the subscription's owner.
+owner = object()
+
+subscribe_to = bpy.context.object.location
+
+def msgbus_callback(*args):
+    # This will print:
+    # Something changed! (1, 2, 3)
+    print("Something changed!", args)
+
+bpy.msgbus.subscribe_rna(
+    key=subscribe_to,
+    owner=owner,
+    args=(1, 2, 3),
+    notify=msgbus_callback,
+)