Mention is default for MMD resolution

Glossary isn't the best place, but it's the only one that shows up for `multi-dispatch` search

doc/Language/glossary.pod6

@@ -584,7 +584,8 @@ X<|Multi-Dispatch>X<|MMD>
 
 The process of picking a candidate for calling of a set of methods or subs that
 come by the same name but with different arguments. The most narrow candidate
-wins.
+wins. In case of an ambiguity, a routine with C<is default> trait will be
+chosen if one exists, otherwise an exception is thrown.
 
 =head1 multi-method
 X<|multi-method>