Add operator declarations to Sub.

Actually, they were pretty well covered elsewhere, so it's basically a
set of pointers. Closes #1573

Author: JJ

doc/Type/Sub.pod6

@@ -40,6 +40,15 @@ C<$objectioner> will contain the C<can-be-seen> subroutine, even if it has been
 declared in another scope; calling it with C<42> will return C<"Really?"> with
 the number 42 mixed in, as shown in the last sentence.
 
+=head1 Operators
+
+Operators are also C<Sub>s. Their definition includes
+L<the category they belong to|/language/operators#Operator_classification>
+and their
+L<code, precedence and associativity|/language/functions#Defining_operators>.
+The syntax used in their definition is an example of
+L<extended identifiers|/language/syntax#Extended_identifiers>.
+
 =head1 Traits
 X<|trait_mod (declarator)>