mountd.8: Document the new -A mountd option

Commit fefb7c399b39 added warning messages noting
that administrative controls that exported directories
that are not local server file system mount points actually
export the entire local server file system.
This commit also added a new command line option "-A' that
silences these warnings.

This patch documents the new "-A' mountd option.

This is a content change.

Reviewed by: markj, pauamma_gundo.com (manpages)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D44692