ctime.3: remove warning about tm_zone and tm_gmtoff

These are standardized in posix 2024 via Austin Group Defect 1533. So they
won't go away or change anytime soon.

ok job millert

lib/libc/time/ctime.3

@@ -1,7 +1,7 @@
-.\"	$OpenBSD: ctime.3,v 1.53 2026/05/29 06:33:12 job Exp $
+.\"	$OpenBSD: ctime.3,v 1.54 2026/05/31 15:21:04 tb Exp $
 .\"
 .\"
-.Dd $Mdocdate: May 29 2026 $
+.Dd $Mdocdate: May 31 2026 $
 .Dt CTIME 3
 .Os
 .Sh NAME
@@ -244,19 +244,15 @@ includes the following fields:
 	const char *tm_zone;	/* abbreviation of timezone name */
 .Ed
 .Pp
-The
 .Fa tm_zone
 and
 .Fa tm_gmtoff
-fields exist, and are filled in by
+are filled in by
 .Fn mktime ,
 .Fn localtime ,
 .Fn timegm ,
 and
-.Fn gmtime ,
-but are not standardized.
-There is no guarantee that these fields will continue to exist
-in this form and they may be altered or removed in a future release.
+.Fn gmtime .
 .Pp
 .Fa tm_isdst
 is non-zero if summer time is in effect.