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