Mention that enum constructors are functions

[steveklabnik]

Fixes #25850

[rust-highfive]

r? @pcwalton

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[steveklabnik]

r? @alexcrichton

[Gankra]

Strings seem distracting in this example. &'static str or i32 might keep this simpler unless there's a reason to avoid them?

[steveklabnik]

I kept Strings because they're used in the enum above, specifically to avoid dealing with lifetime and copy issues.

[Gankra]

@bors r+ rollup

[bors]

📌 Commit b242a86 has been approved by Gankro