nfsd.8: Document ways to minimize Copy operation times

For NFSv4.2, a Copy operation can take a long time to complete.
If there is a concurrent ExchangeID or DelegReturn operation
which requires the exclusive lock on all NFSv4 state, this can
result in a stall of the nfsd server.

This patch documents ways to avoid this problem.

This is a content change.

Reviewed by: karels, wollman, pauamma_gundo.com (manpages)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D44395