Fix the rule introduced by #176 and accord doc with rustc-guide

[scalexm]

The rule introduced in #176 was too restrictive, as can be seen with this example:

trait Bar { }

trait Baz { }
// Implemented(Self: Baz) :- FromEnv(Self: Baz).

trait Foo {
    type Item<T>: Baz where T: Bar;
}

// (*) FromEnv(<Self as Foo>::Item<T>: Baz) :- FromEnv(Self: Foo), FromEnv(T: Bar).

struct AlwaysBar<T> { }
impl<T> Bar for AlwaysBar<T> { }

fn baz<X: Baz>() { }

fn foo<X: Foo, T>() // where AlwaysBar<T>: Bar
{
    // We must prove: `Implemented(<X as Foo>::Item<AlwaysBar<T>>: Baz)`.
    // The only way to prove it is to use the rule (*).
    // This requires `FromEnv(Always<T>: Bar)`, which we don't have in our env.
    baz::<X::Item<AlwaysBar<T>>>()
}

So we change it to be:

FromEnv(<Self as Foo>::Item<T>: Baz) :- FromEnv(Self: Foo), Implemented(T: Bar).

(I also accorded the doc with rust-lang/rustc-dev-guide#210)