vidcontrol: be explicit that -p and -P are sc(4) only

PR:		263099
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation

usr.sbin/vidcontrol/vidcontrol.1

@@ -13,7 +13,7 @@
 .\"     @(#)vidcontrol.1
 .\" $FreeBSD$
 .\"
-.Dd October 20, 2018
+.Dd April 6, 2022
 .Dt VIDCONTROL 1
 .Os
 .Sh NAME
@@ -388,12 +388,16 @@ For details about that
 format see
 .Sx Format of Video Buffer Dump
 below.
+Supported only with
+.Xr syscons 4 .
 .It Fl P
 Same as
 .Fl p ,
 but dump contents of the video buffer in a plain text format
 ignoring nonprintable characters and information about text
 attributes.
+Supported only with
+.Xr syscons 4 .
 .It Fl H
 When used with
 .Fl p