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

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
Differential Revision:	https://reviews.freebsd.org/D37284

(cherry picked from commit 8a220816c72261efce304568498279fdda6b57cb)