Slight touchups to grapheme break generator.

Forgot to add a note about the documentation, and to make the script
executable like planned.

Author: ShimmerFairy

tools/GraphemeBreakTestGen.raku

@@ -187,4 +187,6 @@ sub MAIN(IO(Str) $gbt-file #=(Pathname for the GraphemeBreakTest.txt file, from
         $from = $tdata.write-file($path, $from, 200);
         $file-idx++;
     }
+
+    say "Don't forget to update docs/unicode-generated-tests.asciidoc to indicate that you've updated this and other test files!";
 }