add operator classifications to the Index

In the spirit of #728, adding operator classifications to the index.

Author: molecules

doc/Language/operators.pod6

@@ -77,6 +77,11 @@ In the operator descriptions below, a default associativity of I<left>
 is assumed.
 
 =head1 Operator classification
+X<|prefix operator>
+X<|infix operator>
+X<|postfix operator>
+X<|circumfix operator>
+X<|postcircumfix operator>
 
 Operators can occur in several positions relative to a term: