Update auto traits in the unstable book

Add a `?DynSized` unbound to the auto trait in the example, and explain
why it is necessary

Author: mikeyhew

src/doc/unstable-book/src/language-features/optin-builtin-traits.md

@@ -19,12 +19,22 @@ has explictly opted out via a negative impl.
 impl !Type for Trait
 ```
 
+[#46108] added the `DynSized` trait, which is an implicit bound for all traits. Auto traits may not
+have bounds, so you must explicitly remove this bound with `?DynSized`. [#44917] adds a `Move` trait
+which is also implicit, so when that lands, you will have to add `?Move` as well.
+
+[#46108]: https://github.com/rust-lang/rust/pull/46108
+[#44917]: https://github.com/rust-lang/rust/pull/44917
+
+
 Example:
 
 ```rust
-#![feature(optin_builtin_traits)]
+#![feature(optin_builtin_traits, dynsized)]
+
+use std::marker::DynSized;
 
-auto trait Valid {}
+auto trait Valid: ?DynSized {}
 
 struct True;
 struct False;
@@ -38,7 +48,6 @@ fn must_be_valid<T: Valid>(_t: T) { }
 fn main() {
     // works
     must_be_valid( MaybeValid(True) );
-                
     // compiler error - trait bound not satisfied
     // must_be_valid( MaybeValid(False) );
 }