namei: Update documentation.

This should have been done 2+ years ago when the td argument was dropped
from NDINIT() and the NDFREE() macro and the SAVENAME and SAVESTART
flags were retired.

MFC after: 3 days
Fixes: 7e1d3eefd410 ("vfs: remove the unused thread argument from NDINIT*")
Fixes: 269c564b90d3 ("vfs: retire NDFREE")
Sponsored by: NetApp, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D50375

(cherry picked from commit ee84b4e2fdf09f9a6dfa43e0931d8671cc15ebf9)