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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
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