Document trap on set subroutines on Associative

Fixes rakudo/rakudo#2604

Author: Kaiepi

doc/Language/traps.pod6

@@ -152,6 +152,31 @@ Alternatively, you can also use the L<concatenation operator|/routine/~> instead
     sub count-it { say "Count is " ~ $++ }
     =end code
 
+=head2 Using set subroutines on Associative when the value is falsey
+
+Using L<(cont)|/routine/(cont)>, L<∋|/routine/∋>, L<∌|/routine/∌>,
+L<(elem)|/routine/(elem)>, L<∈|/routine/∈>, or L<∉|/routine/∉> on classes
+implementing L<Associative|/type/Associative> will return C<False> if the value
+of the key is falsey:
+
+    =begin code
+    enum Foo «a b»;
+    say Foo.enums ∋ 'a';
+
+    # OUTPUT:
+    # False
+    =end code
+
+Instead, use C<:exists>:
+
+    =begin code
+    enum Foo «a b»;
+    say Foo.enums<a>:exists;
+
+    # OUTPUT:
+    # True
+    =end code
+
 =head1 Blocks
 
 =head2 Beware of empty "blocks"