Fixup: use \impldef for implementation-defined

Author: jensmaurer

source/preprocessor.tex

@@ -621,8 +621,10 @@
 \pnum
 If the header identified by the \grammarterm{header-name}
 denotes an importable header\iref{module.import},
-it is implementation-defined whether
-the preprocessing directive
+it is
+\impldef{replacing source file inclusion of importable header
+with \tcode{import}}
+whether the preprocessing directive
 is instead replaced by the \grammarterm{preprocessing-token}{s}
 \begin{ncbnf}
 \terminal{import} header-name \terminal{;}