Add deprecation notice to snd_maestro(4).

As suggested in:
https://wiki.freebsd.org/WhatsGoing/FreeBSD13

this old driver is buggy and no one is working on it so we should deprecate
it for the next release.

MFC after:	3 days
Diferential Revision:	https://reviews.freebsd.org/D21139

share/man/man4/snd_maestro.4

@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd December 15, 2005
+.Dd August 12, 2019
 .Dt SND_MAESTRO 4
 .Os
 .Sh NAME
@@ -73,6 +73,9 @@ The
 .Nm
 device driver first appeared in
 .Fx 4.2 .
+.Pp
+This driver will be removed in a future version of the
+.Fx base system.
 .Sh AUTHORS
 This manual page was written by
 .An Jorge Mario G. Mazo Aq Mt jgutie11@eafit.edu.co .