feat: clarify interaction between nullability binding and any type parameters

[benbellick]

• Document how nullability mode (MIRROR, DECLARED_OUTPUT, DISCRETE) interacts with any[\d] type parameter binding
• Add concrete examples showing outermost vs inner nullability, compound types, cross-structural binding, and DISCRETE matching

Closes #943

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This change is 

[benbellick]

I am aware of the complexity added here. If someone can think of a better way to manage this complexity, that would be much preferred. The fundamental problem is dealing with how nullabilityHandling interacts with binding to any type parameters. #949 clarified that with nullability handling MIRROR and DECLARED_OUTPUT, the formal parameters must not have nullability markers. However, nothing stops us from passing in arguments that contain nullability arguments.

This PR is a stab at addressing this interaction.

There is also the case of DISCRETE which is accounted for here. The approach is to just say that the outermost nullability must be declared explicitly and must match when invoked.

[Slimsammylim]

The explanations and examples here are very clear and make sense to me. My only nit is I prefer if the all the reason boxes are filled, but that's just because I like when explanations are obvious. Either way is good!

[benbellick]

@Slimsammylim I added them, good suggestion!