Update sealed traits pattern

src/future-proofing.md

@@ -13,10 +13,6 @@ non-breaking way by using the sealed trait pattern.
 pub trait TheTrait: private::Sealed {
     // Zero or more methods that the user is allowed to call.
     fn ...();
-
-    // Zero or more private methods, not allowed for user to call.
-    #[doc(hidden)]
-    fn ...();
 }
 
 // Implement for some types.
@@ -25,19 +21,24 @@ impl TheTrait for usize {
 }
 
 mod private {
-    pub trait Sealed {}
+    pub trait Sealed {
+        // Zero or more private methods, not allowed for user to call.
+        fn ...();
+    }
 
     // Implement for those same types, but no others.
-    impl Sealed for usize {}
+    impl Sealed for usize {
+        /* ... */
+    }
 }
 ```
 
-The empty private `Sealed` supertrait cannot be named by downstream crates, so
-we are guaranteed that implementations of `Sealed` (and therefore `TheTrait`)
-only exist in the current crate. We are free to add methods to `TheTrait` in a
+The private `Sealed` supertrait cannot be named by downstream crates, so we are
+guaranteed that implementations of `Sealed` (and therefore `TheTrait`) only
+exist in the current crate. We are free to add items to `TheTrait` in a
 non-breaking release even though that would ordinarily be a breaking change for
-traits that are not sealed. Also we are free to change the signature of methods
-that are not publicly documented.
+traits that are not sealed.  We are also free to change the signature of
+`Sealed` in any way, as its items are not visible to downstream crates.
 
 Note that removing a public method or changing the signature of a public method
 in a sealed trait are still breaking changes.