Fix documentation lints

[ia0]

No description provided.

[netlify]

✅ Deploy Preview for cute-starship-2d9c9b canceled.

Name	Link
🔨 Latest commit	8b64a7d
🔍 Latest deploy log	https://app.netlify.com/sites/cute-starship-2d9c9b/deploys/66423c584fc98a0008007ab6

[ia0]

This actually depends on #150 to pass CI.

[jamesmunns]

CC @ia0, I'm not actually sure what the failing line is supposed to do, it was introduced in #108 by @dtolnay, it's possible this is only intended to be activated in docs-rs with a nightly build, if so should we avoid activating this in CI?

[jamesmunns]

This is the failing line btw, it is complaining about the use of #![feature(...)] on stable:

postcard/src/lib.rs

Line 4 in b7f67b0

#![cfg_attr(docsrs, feature(doc_cfg))]

[ia0]

The last commit should fix it. We should indeed only test the documentation in nightly. Also regarding the doc_cfg feature, we could probably simplify the code by using auto_doc_cfg instead. This could be done in another PR (after #151).