Guide: make less ignores

[steveklabnik]

as @huonw points out here: https://github.com/rust-lang/rust/pull/16206/files#r15733110

I didn't realize that I could do this for examples, and so many more of them can become non-ignore. After the full first draft is up, each ignored example should get a review to see if it can be made non-ignore.

[steveklabnik]

Also should_fail is probably better than ignore on most of these. I suck at rustdoc.

[steveklabnik]

I haven't been able to get should_fail to work :/