docs: don't try to get comments from rcupdate_wait.h or rcutree.h

Neither file contains any kerneldoc comments, so including them generates
these warnings in the docs build:

  ./include/linux/rcupdate_wait.h:1: warning: no structured comments found
  ./include/linux/rcutree.h:1: warning: no structured comments found

Remove them and make life a little quieter.

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