MFC 253656: Enhance the description of NOTE_TRACK:

MFC 253656: Enhance the description of NOTE_TRACK:
- NOTE_TRACK has never triggered a NOTE_TRACK event from the parent pid.
  If NOTE_FORK is set, the listener will get a NOTE_FORK event from
  the parent pid, but not a separate NOTE_TRACK event.
- Explicitly note that the event added to monitor the child process
  preserves the fflags from the original event.
- Move the description of NOTE_TRACKERR under NOTE_TRACK as it is not a
  bit for the user to set (which is what this list pupports to be).
  Also, explicitly note that if an error occurs, the NOTE_CHILD event
  will not be generated.

git-svn-id: svn://svn.freebsd.org/base/stable/8@257717 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f