Rework documentation of OLD_*.

• Be more explicit in the difference between OLD_DIRS and OLD_FILES (the former is only in delete-old-libs whereas the latter is in delete-old).

• Document that debug symbols in /usr/lib/debug/ for files in OLD_FILES and OLD_LIBS are removed as well.

Reviewed by: emaste
Sponsored by: The University of Cambridge, Google Inc.
Differential Revision: https://reviews.freebsd.org/D33847