PM / Runtime: Document power.runtime_auto and related functions

The power.runtime_auto device flag and the helper functions
pm_runtime_allow() and pm_runtime_forbid() used to modify it are a
part of the run-time power management framework and therefore they
should be described in Documentation/power/runtime_pm.txt.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>