doc: debugobjects: actually pull in the kerneldoc comments

Add the appropriate markup to get the kerneldoc comments out of
lib/debugobjects.c that have never seen the light of day until now.

A logical next step, left for the reader at the moment, is to move the
function descriptions *out* of debug-objects.rst and into the kerneldoc
comments themselves.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>