nfsstat: update option strings in docs

Add the missing -q option to the nfsstat(1) manpage SYNOPSIS (it is
already documented in DESCRIPTION), and add the missing -E and -q
options to the built-in usage output.

PR: 275912
Pull Request: https://github.com/freebsd/freebsd-src/pull/958

(cherry picked from commit 30ce26cacfd626552c647c9327dd510e7f421d97)