augment comment



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@4939 a5019735-40e9-0310-863c-91ae7b9d1cf9
diff --git a/include/pub_tool_tooliface.h b/include/pub_tool_tooliface.h
index d9d4972..9796a3d 100644
--- a/include/pub_tool_tooliface.h
+++ b/include/pub_tool_tooliface.h
@@ -262,7 +262,9 @@
 /* Does the tool need shadow memory allocated? */
 extern void VG_(needs_shadow_memory)( void );
 
-/* Does the tool replace malloc() and friends with its own versions? */
+/* Does the tool replace malloc() and friends with its own versions?
+   This has to be combined with the use of a vgpreload_<tool>.so module
+   or it won't work.  See massif/Makefile.am for how to build it. */
 // The 'p' prefix avoids GCC complaints about overshadowing global names.
 extern void VG_(needs_malloc_replacement)(
    void* (*pmalloc)               ( ThreadId tid, SizeT n ),