Rust API Guideline: Crate sets `html_root_url` attribute

[clarfonthey]

We shouldn't do this for crates which have documentation turned off in Cargo.toml.

[mcarton]

Can you provide some information about this attribute? I don't know what that would be necessary (don't even know what that does).
I see it's a guideline, but there is no rationnal and it is merely mentionned in the book.

[clarfonthey]

@mcarton this provides a canonical URL where rustdoc can redirect URLs for a crate. It's how links to libstd get redirected to doc.rust-lang.org, and it makes it easier to allow interop between rustdocs without having to include local docs for dependency crates all the time.

[mcarton]

@dtolnay has open an issue on the guidelines to document that one better rust-lang/api-guidelines#75 ❤️

[jyn514]

See rust-lang/api-guidelines#229 for more information about this.