Add comment about x, y int being a single ResultVariableDecl

Author: owen-mc

go/ql/lib/semmle/go/Decls.qll

@@ -667,6 +667,10 @@ class ReceiverDecl extends FieldBase, Documentable, ExprParent {
  * func f4(input string) (output string, err error) { return}
  * func f5(e error) (x, y int) { return }
  * ```
+ * 
+ * Note: `x, y int` is a single `ResultVariableDecl` even though it declares
+ * two different result variables. Use the member predicate `getTypeExpr()` to
+ * get `int`, `getNameExpr(0)` to get `x` and `getNameExpr(1)` to get `y`.
  */
 class ResultVariableDecl extends ParameterOrResultDecl {
   ResultVariableDecl() { rawIndex < 0 }