Document -w flag is an extension to POSIX.

PR:		201937
Submitted by:	<fullermd AT over-yonder DOT net>
Approved by:	bcr (mentor)
Differential Revision:	https://reviews.freebsd.org/D11842

usr.bin/cut/cut.1

@@ -31,7 +31,7 @@
 .\"     @(#)cut.1	8.1 (Berkeley) 6/6/93
 .\" $FreeBSD$
 .\"
-.Dd August 8, 2012
+.Dd August 3, 2017
 .Dt CUT 1
 .Os
 .Sh NAME
@@ -154,6 +154,10 @@ The
 .Nm
 utility conforms to
 .St -p1003.2-92 .
+.Pp
+The
+.Fl w
+flag is an extension to the specification.
 .Sh HISTORY
 A
 .Nm