Documentation: media: camera-sensor: Improve power management documentation

Camera sensor drivers are highly subject to cargo cult programming, with
back practices being copied from old to new drivers. In particular, many
drivers implement system and runtime PM incorrectly. As a first step
towards fixing this situation, refactor and expand the power management
documentation to detail correct usage of system and runtime PM.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>