unified/doc: Revise doxygen for kernel APIs

Does a general cleanup including:

* adding descriptions for APIs that are missing them
* adding information that would be helpful to users
  and removing information that wouldn't be helpful
* correcting errors
* aligning the terminology with the terminology used
  in the Kernel Primer document
* standardizing the way information is presented

Change-Id: I536644a7dc60b62100e379a199a645344430beb7
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>