truncate(2): extending the file is required by POSIX 2008

Update the man page to mention that extending a file with truncate(2)
is required by POSIX as of 2008.

Reviewed by:	bcr
MFC after:	2 weeks
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D23354

lib/libc/sys/truncate.2

@@ -28,7 +28,7 @@
 .\"     @(#)truncate.2	8.1 (Berkeley) 6/4/93
 .\" $FreeBSD$
 .\"
-.Dd May 4, 2015
+.Dd January 24, 2020
 .Dt TRUNCATE 2
 .Os
 .Sh NAME
@@ -160,6 +160,9 @@ system calls appeared in
 These calls should be generalized to allow ranges
 of bytes in a file to be discarded.
 .Pp
-Use of
+Historically, the use of
 .Fn truncate
-to extend a file is not portable.
+or
+.Fn ftruncate
+to extend a file was not portable, but this behavior became required in
+.St -p1003.1-2008 .