PM: runtime: Fix pm_runtime_active() kerneldoc comment

The kerneldoc comment of pm_runtime_active() does not reflect the
behavior of the function, so update it accordingly.

Fixes: 403d2d11 ("PM: runtime: Add kerneldoc comments to multiple helpers")
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>