Add nullfs specific nocache option.

sbin/mount_nullfs/mount_nullfs.8

@@ -33,7 +33,7 @@
 .\"     @(#)mount_null.8	8.6 (Berkeley) 5/1/95
 .\" $FreeBSD$
 .\"
-.Dd October 3, 2016
+.Dd April 22, 2022
 .Dt MOUNT_NULLFS 8
 .Os
 .Sh NAME
@@ -80,6 +80,12 @@ flag followed by a comma separated string of options.
 See the
 .Xr mount 8
 man page for possible options and their meanings.
+Additionally the following option is supported:
+.Bl -tag -width indent
+.It Cm nocache
+Disable caching in the null layer.
+Some lower-layer file systems may force this option.
+.El
 .El
 .Pp
 The null layer has two purposes.