rmdir.1: Document non-POSIX -v option

MFC after:		3 days
Reviewed by:		ziaee
Differential Revision:	https://reviews.freebsd.org/D57675

bin/rmdir/rmdir.1

@@ -29,7 +29,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd March 15, 2013
+.Dd June 19, 2026
 .Dt RMDIR 1
 .Os
 .Sh NAME
@@ -104,6 +104,10 @@ The
 utility is expected to be
 .St -p1003.2
 compatible.
+.Pp
+The
+.Fl v
+option is an extension to that specification.
 .Sh HISTORY
 A
 .Nm