mountd.8: Document the new -A mountd option

Commit fefb7c3 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

usr.sbin/mountd/mountd.8

@@ -25,7 +25,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd March 18, 2023
+.Dd April 8, 2024
 .Dt MOUNTD 8
 .Os
 .Sh NAME
@@ -35,7 +35,7 @@
 mount requests
 .Sh SYNOPSIS
 .Nm
-.Op Fl 2delnRrS
+.Op Fl 2AdelnRrS
 .Op Fl h Ar bindip
 .Op Fl p Ar port
 .Op Ar exportsfile ...
@@ -60,6 +60,16 @@ Allow the administrator to force clients to use only the
 version 2
 .Tn NFS
 protocol to mount file systems from this server.
+.It Fl A
+Silence the warnings related to
+.Dq administrative controls .
+These warnings remind users that an exported
+.Dq administrative control
+directory that is not a local server file system mount point
+actually exports the entire local file system and not just
+the subtree below the directory exported.
+(See
+.Xr exports 5 )
 .It Fl d
 Output debugging information.
 .Nm