[assertions.assert] Add ISO C reference for standard error stream

Author: frederick-vs-ja

source/diagnostics.tex

@@ -482,7 +482,7 @@
 If the evaluation yields \tcode{true} there are no further effects.
 \item
 Otherwise, the \tcode{assert} macro's expression
-creates a diagnostic on the standard error stream in an
+creates a diagnostic on the standard error stream (\IsoC{}, 7.23.3) in an
 \impldef{format of diagnostic created by \tcode{assert} macro's expression}
 format and calls \tcode{abort()}.
 The diagnostic contains \tcode{\#}\mname{VA_ARGS} and