PM: runtime: Add kerneldoc comments to multiple helpers

Add kerneldoc comments to multiple PM-runtime helper functions
defined as static inline wrappers around lower-level routines to
provide quick reference decumentation of their behavior.

Some of them are similar to each other with subtle differences only
and the behavior of some of them may appear as counter-intuitive, so
clarify all that to avoid confusion.

Signed-off-by: Rafael J. Wysocki <[email protected]>
Acked-by: Alan Stern <[email protected]>
Reviewed-by: Sakari Ailus <[email protected]>