Docs for build script `cargo:rustc-cfg` are unclear

[Lokathor]

In the Build Scripts page the docs for cargo:rustc-cfg give it as rustc-cfg=FEATURE, but apparently the correct form is to use cargo:rustc-cfg=feature=\"feature_name\" , so perhaps we should update that page.

[sfackler]

rustc-cfg=FEATURE will enable #[cfg(FEATURE)].

[Lokathor]

ooooooooooooh

Yeah that's not clear in the docs.