PM / runtime: Document steps for device removal

Put a reminder that during device removal drivers should revert all PM
runtime changes from the probe.

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