Add section about lib function visibility in macro

[straight-shoota]

cf. crystal-lang/crystal#12848

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[beta-ziliani]

It's a bit odd that we're saying this about libs, but not about other types. But I guess it's better to have something somewhere than anything, so OK.

[beta-ziliani]

I don't think it's clear what "anywhere" means here. Also, probably we want to link to the API.

Suggested change

	Lib functions are visible in the macro language anywhere in the program.
	Lib functions are visible in the macro language using the method [`TypeNode#methods`](https://crystal-lang.org/api/1.6.2/Crystal/Macros/TypeNode.html#methods%3AArrayLiteral%28Def%29-instance-method):

[straight-shoota]

"anywhere" might not be very clear, but it's important. TypeNode#methods for other types only works within method context. The point we're mentioning this here is that for lib types it also works outside of methods.

[beta-ziliani]

but if it's important, it should be clear. anyway, we can leave it for a future improvement.