update documentation of tuple/unit structs

I made the "tuple structs are useless" editorializing a bit weaker and moved it to the end. Feel free to overrule me on that.

I also added an example of how to unpack a tuple struct with dot notation, because it came up on IRC.

`braced_empty_structs` is stable now, so I updated the example for unit-like structs to use that syntax. Should we show both ways?

cc @ubsan
r? @steveklabnik or @GuillameGomez

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