[module.import] Clarify that only header imports make macros visible.

Author: jensmaurer

source/modules.tex

@@ -497,6 +497,8 @@
 preprocessor, and results in macros defined at the
 end of phase 4 of translation of the header unit
 being made visible as described in \ref{cpp.import}.
+Any other \grammarterm{module-import-declaration}
+does not make macros visible.
 \end{note}
 
 \pnum