[handler.functions] Add cross-reference to [intro.races] (#6845)

Author: eisenwave

source/lib-intro.tex

@@ -3340,8 +3340,8 @@
 \end{itemize}
 
 \pnum
-Calling the \tcode{set_*} and \tcode{get_*} functions shall not incur a data race. A call to
-any of the \tcode{set_*} functions shall synchronize with subsequent calls to the same
+Calling the \tcode{set_*} and \tcode{get_*} functions shall not incur a data race\iref{intro.races}.
+A call to any of the \tcode{set_*} functions shall synchronize with subsequent calls to the same
 \tcode{set_*} function and to the corresponding \tcode{get_*} function.
 
 \rSec3[res.on.functions]{Other functions}