PM: runtime: Improve kerneldoc of pm_runtime_get_if_active()

The kerneldoc comment of pm_runtime_get_if_active() doesn't list the
second argument of the function properly, so fix that and while at it
clarify that comment somewhat and add some markup to it.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>