Note down some semantic restrictions on dyn-Trait and impl-Trait

Furthermore, drop rule `type.trait-object.name` entirely
and parts of `type.trait-object.bounds` since they mixed
syntax & semantics and tried to redescribe the grammar in
prose which didn't add value, were too ambiguous & incomplete.

Author: fmease

src/types/impl-trait.md

@@ -23,6 +23,10 @@ fn foo(arg: impl Trait) {
 fn bar() -> impl Trait {
 }
 ```
+
+r[type.impl-trait.bounds]
+There must be at least one trait bound, no more than one `use<..>` bound, and no more than one opt-out bound (e.g., `?Sized`).
+
 r[type.impl-trait.param]
 ## Anonymous type parameters
 

src/types/trait-object.md

@@ -14,11 +14,8 @@ A *trait object* is an opaque value of another type that implements a set of tra
 r[type.trait-object.impls]
 Trait objects implement the base trait, its auto traits, and any [supertraits] of the base trait.
 
-r[type.trait-object.name]
-Trait objects are written as the keyword `dyn` followed by a set of trait bounds, but with the following restrictions on the trait bounds.
-
-r[type.trait-object.constraint]
-There may not be more than one non-auto trait, no more than one lifetime, and opt-out bounds (e.g. `?Sized`) are not allowed. Furthermore, paths to traits may be parenthesized.
+r[type.trait-object.bounds]
+There must be at least one trait bound, there may not be more than one non-auto trait, no more than one lifetime, and opt-out bounds (e.g., `?Sized`) and `use<..>` bounds are not allowed.
 
 For example, given a trait `Trait`, the following are all trait objects: