Spec: restrict the type returned by should_not be_nil and others

[asterite]

Fixes #8369

Follow up to #8240

[straight-shoota]

This is technically missing should be_nil. I assume that's on purpose because it's hard to think of a use case for this?

[straight-shoota]

I'd avoid placing an example block inside a sentence.

Suggested change

	# This overload will be called when doing:
	# This overload accepts a `be_a(Type)` expectation and makes sure, at compile-time, that the returned object is of type `Type`.

[straight-shoota]

Any reason for not mentioning the be_a method here? It would make the documentation more accessible for readers familiar with the DSL.

[asterite]

Sorry, what do you mean? I changed the code example and be_a is used there.

[straight-shoota]

I meant mentioning it in the text.

But yeah, having it in the sample is probably fine.

[asterite]

This is technically missing should be_nil. I assume that's on purpose because it's hard to think of a use case for this?

Right, the resulting type will always be nil and afterwards there's not much you can do with that.

[asterite]

Fixed once more. Let me know if this is good now.