Use doc_auto_cfg feature instead of ad-hoc #[doc] comments.

[glandium]

docs.rs uses the nightly compiler, allowing to enable the doc_auto_cfg,
which automatically documents the features required for each documented
item.

Before:
$ du -sh crates/libs/{sys,windows}
32M crates/libs/sys
178M crates/libs/windows

After:
$ du -sh crates/libs/{sys,windows}
21M crates/libs/sys
164M crates/libs/windows

While here, add the missing all-features setting for docs.rs for the
windows crate.

[glandium]

Waw, peak rustdoc memory usage for the windows crate is 11.5G... (and a very large runtime)

[rylev]

If the final render seen on docs.rs is exactly equivalent before and after this change, then I think it's obvious to accept this change, but I doubt that's the case. Can you take a look at the output when running cargo doc and summarize the differences?

[glandium]

Before:



After:

The main difference is the "Required features" not being shown on each item in the module, but that's because they are the same as the one listed for the module. In a module where extra features are required, they're shown:

I'd argue the new output is better.

[riverar]

This looks worse in every way possible. It's a distracting color, verbose due to conjunction (and) use, and can't be cut/pasted into a Cargo.toml. Can those issues be resolved? If not, I don't think they'll accept this change.

[glandium]

It's also what's used in the rust ecosystem: https://docs.rs/tokio/1.20.1/tokio/macro.join.html

[glandium]

Can those issues be resolved?

Issues can be filed against rustdoc.

[riverar]

Will leave this open until Kenny returns but it's not likely to get merged in. Would recommend holding on any further commits.

[kennykerr]

Thanks for the contribution! Sorry I've been away for a few weeks. In future, please open an issue before creating a PR. We have discussed this before at length. The rustdoc feature support is very limited in many ways and lacks the support we need to provide high-quality feature documentation. While I would also prefer to use the built-in support, the rustdoc team hasn't made any progress on the various issues we encountered so I'm unlikely to revert the custom doc generation. I can try to dig up the old issues where this was discussed if you'd like to discuss this further.