ignore X<> markers.

Author: coke

xt/rakudoc-types.rakutest

@@ -43,6 +43,7 @@ sub render-node($node) {
 sub is-valid-type($node, $parent, $file) {
     # only care about I<>, C<>, L<>, etc.
     return unless $node ~~ Pod::FormattingCode;
+    return if $node.type eq 'X'; # These are OK as is, and not user-visible
 
     # Does this match a documented type?
     my $type = $node.contents.join('');