Allow docs.rs to auto-label optional features.

[ripytide]

See rustdoc documentation on it at https://doc.rust-lang.org/rustdoc/unstable-features.html#doc_auto_cfg-automatically-generate-doccfg

This should fix #34

[kornelski]

This seems to be poorly implemented — the labels are more verbose and contain cfg syntax. doc(cfg) produces nicer output.

[ripytide]

I'm confused by what you mean by verbose?

This is what I see using #![feature(doc_auto_cfg)] for the GRB page:

Vs without:

[kornelski]

Examples in the linked rustdoc issue looked worse and printed raw cfg expressions. Maybe it's been fixed since?

[ripytide]

Oh I see the issue in rust-lang/rust#43781, yeah the example given there does look quite verbose but I think they've mostly fixed it by now.

Here is the listing page which I think is very manageable: