mention stdatomic(3)

Author: Vladimir Kotal

threads.tex

@@ -1283,7 +1283,8 @@
 \item The atomic primitives and types in C11 is an optional feature, so should
 be wrapped under \texttt{\_\_STDC\_NO\_ATOMICS\_\_} negative define.
 \item A good description of the C11 atomic API can be found on
-\url{http://en.cppreference.com/w/c/atomic}
+\url{http://en.cppreference.com/w/c/atomic}. Some systems have the
+stdatomic(3) man page.
 \end{itemize}