PM / runtime: Update documentation to reflect the current code flow

The runtime PM documentation in runtime_pm.txt has not been updated
after some changes to the system suspend and resume core code, so
update it to reflect the current code flow.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>