tool: Update a comment ifchange [ci skip]

tool/ifchange

@@ -1,7 +1,8 @@
 #!/bin/sh
 # usage: ifchange target temporary
 
-# Used in generating revision.h via Makefiles.
+# Used in generating various files such as rbconfig.rb, revision.h,
+# etc. via Makefiles.
 
 help() {
     cat <<HELP