intro.3: Document 'libstdthreads' (the ISO C11 '<threads.h>' library).

It's still relatively uncommon to see C11 '<threads.h>' being used,
but it's worth documenting, especially since FreeBSD requires a
different library for C11 threads  compared to glibc (part of '-lpthread')
or musl libc  (included by default).

PR:	267551
Submitted by:	Alois Klink <alois plus freebsd at aloisklink dot com>
Reviewed by:	pauamma
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D37284

share/man/man3/intro.3

@@ -28,7 +28,7 @@
 .\"     @(#)intro.3	8.1 (Berkeley) 6/5/93
 .\" $FreeBSD$
 .\"
-.Dd December 18, 2021
+.Dd November 7, 2022
 .Dt INTRO 3
 .Os
 .Sh NAME
@@ -231,6 +231,13 @@ See
 The POSIX threads library.
 See
 .Xr pthread 3 .
+.It Em libstdthreads
+.Pq Fl l Ns Ar stdthreads
+The ISO C11 standard
+.In threads.h
+library.
+See
+.Xr thrd_create 3 .
 .It Em libsysdecode
 .Pq Fl l Ns Ar sysdecode
 The system argument decoding library.