PM: Runtime PM documentation update

This patch (as1318) updates the runtime PM documentation, adding a
section discussing the interaction between runtime PM and system sleep.

[rjw: Rebased and made it agree with the other updates better.]

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>