Rollup merge of #25348 - geofft:trpl-fix-enums, r=steveklabnik

Rollup merge of #25348 - geofft:trpl-fix-enums, r=steveklabnik

The enums chapter at the moment is ... weird. The examples aren't about enums, they're about structs, and most of the chapter talks about how enums don't support comparison operators by default (which is also true of other compound data types.) I think there was a story here once, but some coherency got lost in refactoring.

There are two preliminary patches here, one to combine the struct and tuple-struct chapters, and one to document unit-like structs, because enum syntax is easier to explain once you have those three. The final patch moves the enum chapter after the struct chapter, and rewrites most of it to talk about enums usefully (including covering matches on enums).

r? @steveklabnik