Add missing ESRCH error description to getthrname(2) man page.

ok claudio

lib/libc/sys/getthrname.2

@@ -1,4 +1,4 @@
-.\"	$OpenBSD: getthrname.2,v 1.3 2025/12/03 17:09:23 kurt Exp $
+.\"	$OpenBSD: getthrname.2,v 1.4 2026/06/12 10:56:23 mvs Exp $
 .\"
 .\" Copyright (c) 2023 Philip Guenther <guenther@openbsd.org>
 .\"
@@ -14,7 +14,7 @@
 .\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 .\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 .\"
-.Dd $Mdocdate: December 3 2025 $
+.Dd $Mdocdate: June 12 2026 $
 .Dt GETTHRNAME 2
 .Os
 .Sh NAME
@@ -97,6 +97,12 @@ The
 argument points to an
 invalid address.
 .El
+.Bl -tag -width Er
+.It Bq Er ESRCH
+The thread
+.Fa tid
+does not exist.
+.El
 .Pp
 In addition,
 .Fn getthrname