Unsized coercion of TyCtor(T) clarity issue

In type-coercions.md, the following is really not clear:

* TyCtor(`T`) to TyCtor(`U`), where TyCtor(`T`) is one of
    - `&T`
    - `&mut T`
    - `*const T`
    - `*mut T`
    - `Box<T>`

    and where `U` can be obtained from `T` by [unsized coercion](#unsized-coercions).

    <!--In the future, coerce_inner will be recursively extended to tuples and
    structs. In addition, coercions from subtraits to supertraits will be
    added. See [RFC 401] for more details.-->

Ideally, the text should not require deciphering. It seems to be redundantly stating that &T can be coerced to &U. However, is it merely stating that &, &mut, *const, *mut, and Box have special permissibility when unsized coercions exists for their argument types.

Start with the unsized coercions section in type-coercions.md and read the quoted TyCtor(T) rule in context. Clarify whether the rule describes coercions through &, &mut, *const, *mut, and Box, so the permitted behavior is unambiguous. Done means the documentation explains the rule without requiring readers to decipher the notation.