clarify the parenthetical notation stability error message

This also calls the right API, which e.g. prevents a suggestion
for #![feature(unboxed_closures)] on stable.

Fixes #26970

src/test/compile-fail/unboxed-closure-feature-gate.rs

@@ -21,7 +21,7 @@ trait Foo<A> {
 
 fn main() {
     let x: Box<Foo(isize)>;
-    //~^ ERROR parenthetical notation is only stable when used with the `Fn` family
+    //~^ ERROR parenthetical notation is only stable when used with `Fn`-family
 
     // No errors with these:
     let x: Box<Fn(isize)>;

src/test/compile-fail/unboxed-closure-sugar-not-used-on-fn.rs

@@ -12,11 +12,11 @@
 // Test that the `Fn` traits require `()` form without a feature gate.
 
 fn bar1(x: &Fn<(), Output=()>) {
-    //~^ ERROR angle-bracket notation is not stable when used with the `Fn` family
+    //~^ ERROR of `Fn`-family traits' type parameters is subject to change
 }
 
 fn bar2<T>(x: &T) where T: Fn<()> {
-    //~^ ERROR angle-bracket notation is not stable when used with the `Fn` family
+    //~^ ERROR of `Fn`-family traits' type parameters is subject to change
 }
 
 fn main() { }