update documentation of tuple/unit structs
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
graydon/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust