Rustdoc should pick up docs for tuple struct and enum fields

[TedDriggs]

Rustdoc should expose doc comments written on tuple fields:

pub struct Foo(
    /// Hello docs
    String
);

Tagging @QuietMisdreavus @GuillaumeGomez based on conversation on IRC.

[hcpl]

This issue also applies to tuple variant fields:

pub enum Bar {
    BarVariant(
        /// Hello docs
        String
    ),
}

[GuillaumeGomez]

It promises to be very interesting from a design point of view. We'll to think about how to render this nicely.