mayac authored Nov 29, 2018 and Ryan Scott committed Nov 29, 2018

Summary:
Fix a bug in commit 12eeb9 which permits the following:

```
class C a where
  type T a b
instance C (Maybe a) where
  type forall a b. T (Maybe a) b = b
```

where instead, the user should write:

```
instance C (Maybe a) where
  type forall b. T (Maybe a) b = b
```

Update the users guide to discuss scoping of type variables in
explicit foralls in type family instances.

Test Plan: validate

Reviewers: bgamari, goldfire, monoidal

Reviewed By: goldfire

Subscribers: monoidal, rwbarton, carter

GHC Trac Issues: #15828

Differential Revision: https://phabricator.haskell.org/D5283