document that impls may be more general than traits when it comes to lifetimes

[nikomatsakis]

As discussed in rust-lang/rust#61888, the current (and intended) behavior is that impls may be "more general" than traits when it comes to lifetimes -- though within some limits.We should document this behavior as intended, although some of the particular limits -- i.e., the distinction between early- and late-bound regions -- is something that we may want to change.