`TcRnUserTypeError` 47403 should be folded into `UserTypeError` 64725

[akrmn]

TcRnUserTypeError (47403) and UserTypeError (64725) really are the same error, but they get triggered in different parts of the compiler. At first I thought one was specifically for constraints and the other one for type families, but it's more subtle than that (in some cases, adding/removing a type signature changes which one you get), but for the user it really doesn't make a difference. If they had the same error code, that wouldn't be a problem. Another alternative would be to allow redirections, so e.g. the link for TcRnUserTypeError (47403) simply redirects to the page for UserTypeError (64725).

[PPKFS]

I think having a "See Also" could be useful in this case. Though, if we can pinpoint exactly when one happens vs the other, that could be worth documenting?