Adds an example which fails with invalid filehandles.

I couldn't find a way to deal with them, as indicated in #96 and
rakudo/rakudo#2666. I asked [this question in
SO](https://stackoverflow.com/questions/54501546/dealing-with-invalid-filehandles-and-maybe-other-invalid-objects-too),
will reopen if there's some answer that does not involve catching the
error. Meanwhile, closes #96

doc/Type/IO/ArgFiles.pod6

@@ -78,6 +78,18 @@ sub MAIN () {
 and, in fact, can't be used to process the arguments in the command line, since,
 in this case, it would result in an usage error.
 
+Bear in mind that C<$*ARGFILES> is going to contain an handle for every argument
+in a command line, even if that argument is not a valid file.
+
+=for code
+for $*ARGFILES.handles -> $fh {
+    say $fh;
+}
+
+That code will fail if any of the arguments is not the valid name of a file. You
+will have to deal with that case at another level, checking that
+L<C<@*ARGS>|/language/variables#index-entry-%40*ARGS> contains valid file names,
+for instance.
 
 =end pod