Mention that you can add auto traits if principal has them as supers

i.e. document the behavior after r-l/r/119338

Author: WaffleLapkin

src/expressions/operator-expr.md

@@ -388,7 +388,8 @@ reference types and `mut` or `const` in pointer types.
 \* or `T` and `V` are unsized types with compatible metadata:
 * both slice metadata (`*[u16]` -> `*[u8]`, `*str` -> `*(u8, [u32])`).
 * both the same trait object metadata, modulo dropping auto traits (`*dyn Debug` -> `*(u16, dyn Debug)`, `*dyn Debug + Send` -> `*dyn Debug`).
-    * **Note**: *adding* auto traits is not allowed (`*dyn Debug` -> `*dyn Debug + Send` is invalid).
+    * **Note**: *adding* auto traits is only allowed if the principal trait has the auto trait as a super trait
+      (given `trait T: Send {}`, `*dyn T` -> `*dyn T + Send` is valid, but `*dyn Debug` -> `*dyn Debug + Send` is not).
     * **Note**: generics (including lifetimes) must match (`*dyn T<'a, A>` -> `*dyn T<'b, B>` requires `'a = 'b` and `A = B`).
 
 \*\* only when `m₁` is `mut` or `m₂` is `const`. Casting `mut` reference to

src/type-coercions.md

@@ -177,6 +177,8 @@ an implementation of `Unsize<U>` for `T` will be provided:
 
 * `dyn T` to `dyn U` when `U` is one of `T`'s [supertraits].
     * This allows dropping auto traits, i.e. `dyn T + Auto` to `dyn U` is allowed.
+    * This allows adding auto traits if the principal trait has the auto trait as a super trait,
+      i.e. given `trait T: U + Send {}`, `dyn T` to `dyn T + Send` or to `dyn U + Send` coercions are allowed.
 
 * `Foo<..., T, ...>` to `Foo<..., U, ...>`, when:
     * `Foo` is a struct.