Flesh out struct keyword docs

The whole keyword docs thing is pretty new in Rust's history and needs
some work before it's a shining gem. Here's hoping I can provide that.

I basically shoved in a bunch of the most important information from the
reference and the book, along with leaving links to both at the end. I
don't think keyword docs need to have complete detail, just all the
broad strokes, so if someone's confused about a usage of a keyword they
can look at the std documentation for that keyword.

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