Scripting msgbus documentation improvements

The documentation on the blender_python_reference/bpy.msgbus.html page is incomplete and begs for further use cases.

For example, are args always a triplet and static in value, or can variables be used (they seem rather useless as shown)?

How does one provide multiple to the key variable?

How does owner affect the nature of events?

Is it possible to only receive events on specific objects?