Add documentation for GHC-45510

[Kleidukos]

@s-and-witch would you mind checking that the explanation is correct?

[int-index]

This discussion prompted me to submit https://github.com/ghc-proposals/ghc-proposals/pull/656. Check out the updated proposal text and examples, I think it explains quite clearly what's actually happening.

[int-index]

Whatever explanation we end up with for GHC-45510, the main ideas that need to be communicated are

• only out-of-scope type variables are implicitly quantified
• with RequiredTypeArguments, all variables are in scope in type/kind signatures, regardless of their namespace
• but the namespace of a variable is decoupled from whether it's actually a type or term variable, so sometimes we end up with term variables accidentally used in type/kind-level contexts, which triggers GHC-45510

[Kleidukos]

Thanks a lot @s-and-witch and @int-index