Add note about a reason to use mount(8) instead of mount_somefs.

Reported and proof-readed by:	pho
Discussed with:	rodrigc
MFC after:	3 days

sbin/mount/mount.8

@@ -522,6 +522,17 @@ is accessible from the mounted file system.
 The minimum permissions for
 the mount point for traversal across the mount point in both
 directions to be possible for all users is 0111 (execute for all).
+.Pp
+Use of the
+.Nm
+is preferred over the use of the file system specific
+.Pa mount_XXX
+commands.
+In particular, the
+.Pa mountd
+gets a SIGHUP signal, that causes an update of the export list,  
+only when the file system is mounted via
+.Nm .
 .Sh HISTORY
 A
 .Nm