Add "infix stopper" to glossary

Inspired by rakudo/rakudo#2115

Author: lizmat

doc/Language/glossary.pod6

@@ -366,6 +366,14 @@ the L<C<use> keyword|/language/modules#use>.
 
 Include functions from a module in the current namespace.
 
+=head1 X<infix stopper|Reference,infix>
+
+An I<infix stopper> is something that tells us the grammar not try to parse
+an infix at that point. Consider say C<"abc42def" ~~ m|\d+|>. Ordinarily,
+a C<|> in a regex is treated as an alternation, but since the infix stopper
+matches the current regex terminator, we don't continue and try to parse an
+infix at that point. The C<|> is thus treated as the end of the regex.
+
 =head1 X<Instance|Reference,instance>
 
 An I<instance> of a class is also called an I<object> in some other