Fix #2412 Document SetHash --

Add examples for removing SetHash elements

doc/Type/SetHash.pod6

@@ -35,15 +35,19 @@ $fruits<apple kiwi> = False, True;
 say $fruits.keys.sort;  # OUTPUT: «kiwi orange peach␤»
 =end code
 
-Here is a convenient shorthand idiom for adding an element to a SetHash:
+Here is a convenient shorthand idiom for adding and removing SetHash elements:
 
 =begin code
 my SetHash $fruits .= new;
 say $fruits<cherry>;      # OUTPUT: «False␤»
 $fruits<cherry>++;
 say $fruits<cherry>;      # OUTPUT: «True␤»
-
 $fruits<apple banana kiwi>»++; # Add multiple elements
+
+$fruits<cherry>--;
+say $fruits<cherry>;      # OUTPUT: «False␤»
+$fruits<banana kiwi>»-- # Remove multiple elements
+
 =end code
 
 =head1 Creating C<SetHash> objects