getdirentries.2: fix for NFS mounts

It was reported that getdirentries(2) was
returning dirents with d_off set to 0 for an NFS
mount.

This is believed to be correct behaviour at
this time (it may change for some NFS mounts
in the future), but is inconsistent with what the
getdirentries(2) man page says.

This patch fixes the man page.

This is a content change.

PR: 253428
Reviewed by: asomers
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D28664