drm/i915: Kerneldoc for intel_runtime_pm.c

I've decided not to document the functions exported to the audio
driver since really, they shouldn't exist ...

v2: Improvements from Imre's review plus a few more spelling fixes
I've spotted.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>