Rollup merge of #122195 - jieyouxu:impl-return-note, r=fmease

Note that the caller chooses a type for type param

```
error[E0308]: mismatched types
  --> $DIR/return-impl-trait.rs:23:5
   |
LL | fn other_bounds<T>() -> T
   |                 -       -
   |                 |       |
   |                 |       expected `T` because of return type
   |                 |       help: consider using an impl return type: `impl Trait`
   |                 expected this type parameter
...
LL |     ()
   |     ^^ expected type parameter `T`, found `()`
   |
   = note: expected type parameter `T`
                   found unit type `()`
   = note: the caller chooses the type of T which can be different from ()
```

Tried to see if "expected this type parameter" can be replaced, but that goes all the way to `rustc_infer` so seems not worth the effort and can affect other diagnostics.

Revives #112088 and #104755.

Author: matthiaskrgr