Fixes feature flag used in generating the docs in the README

[Michael-J-Ward]

I thought I'd take a stab at fixing at this #82, but I ran into a few errors following the instruction in the README.md

The first error and one that is addressed in this pull request is that the feature flag used to generate the docs is incorrect- it should be docs and not docs.rs

$ cargo doc --features docs.rs --open

error: Package `async-std v0.99.8 (/home/mike/workspace/async-std)` does not have these features: `docs.rs`

Perhaps additionally, it should be --features docs,unstable as it is in .travis.yml . Should I add that to the README? https://github.com/async-rs/async-std/blob/master/.travis.yml#L53

Lastly, I had to use the +nightly flag to get it to the docs and example to run. Should I add this or is it assumed that the user would have this configured somewhere?

Thanks!

[ghost]

Lastly, I had to use the +nightly flag to get it to the docs and example to run. Should I add this or is it assumed that the user would have this configured somewhere?

I think it would be good to use +nightly in README.md.

Perhaps additionally, it should be --features docs,unstable as it is in .travis.yml . Should I add that to the README? https://github.com/async-rs/async-std/blob/master/.travis.yml#L53

Maybe the other way around instead. :)

We use #[cfg(any(feature = "unstable", feature = "docs"))] in our code, which means --features docs is enough to render all APIs, even unstable ones. So we can safely remove ,unstable from .travis.yml.

[Michael-J-Ward]

I've added +nightly to the cargo commands in the README and removed the unstable feature flag from travis.yml and github/workflows/ci.yml

It looks like #[cfg(any(feature = "unstable", feature = "docs"))] is used everywhere except for maybe here:

async-std/src/task/mod.rs

Lines 28 to 31 in ddee472

	#[cfg(feature = "unstable")]
	#[cfg_attr(feature = "docs", doc(cfg(unstable)))]
	#[doc(inline)]
	pub use async_macros::ready;

I'm not yet familiar enough to know if that should be changed because it looks it might be handled in another way there.

[ghost]

@Michael-J-Ward That's a good catch! This line:

#[cfg(feature = "unstable")] 

should be replaced with:

#[cfg(any(feature = "unstable", feature = "docs"))]

[ghost]

Thanks!

[ghost]

bors r+

[yoshuawuyts]

bors is hanging, merging this manually!