Add documentation for enum predicate methods

[stephannv]

• Add a "Question methods" section porting from https://crystal-lang.org/api/1.15.0/Enum.html#question-methods

• Add a different example to Methods section because the current example can lead the reader to think that question methods aren't defined by default.

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[straight-shoota]

I changed the target branch to release/1.15 and thus had to revert one commit that it is master but not latest release.

[Fryguy]

Very minor, but would "Predicate methods" be a better title for the section?

[stephannv]

Very minor, but would "Predicate methods" be a better title for the section?

I prefer "Predicate methods" but in https://crystal-lang.org/api/1.15.0/Enum.html#question-methods the used term is "Question methods". So I kept the same name.

[straight-shoota]

I agree that predicate methods is formally a better term. I'd be happy to change it in both places.

Question method also has some benefit as it's very descriptive and easily understandable though 🤔

[ysbaddaden]

I prefer "Predicate Methods".

[straight-shoota]

Let's do predicate methods then!