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.2.py

@@ -0,0 +1,6 @@
+"""
+Some properties are converted to Python objects when you retrieve them. This
+needs to be avoided in order to create the subscription, by using
+``datablock.path_resolve("property_name", False)``:
+"""
+subscribe_to = bpy.context.object.path_resolve("name", False)