GApplication: add notes about GDBus architecture

GApplication: add notes about GDBus architecture

Some people are trying to write code that calls g_application_register()
then checks to see if we became the primary name owner before exporting
objects.  This sort of approach worked with libdbus-1 because method
calls to the freshly-acquired name would not be dispatched until the
application returned to the mainloop.  With GDBus, however, dispatches
can occur at any time (including in the brief space between acquiring
the name and actually registering the object).

Add documentation to make it clear that you should not expect this to
work.