fts: Improve the description of FTS_NOSTAT

Note that we still need to stat directories and the roots.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D57325

(cherry picked from commit dce6aff90b13f30fcb4c1fa263868607ccda8041)