Match CI doc testing with docs.rs.

[xStrom]

• Remove -Zunstable-options -Zrustdoc-scrape-examples from CI. It wasn't even enabled for docs.rs and the examples we have lead to more noise than anything in the docs, so not worth enabling scraping at this point.
• Treat doc warnings as errors. Historically we've not done so due to various rustdoc bugs giving false positives but I got it to pass without failure right now, so perhaps better times have arrived.
• Pass --all-features at docs.rs to match our CI and reduce the maintenance burden of manually syncing the features list.
• Target only x86_64-unknown-linux-gnu on docs.rs as we don't have any platform specific docs.

[waywardmonkeys]

You snuck a typos update in as well!

For other repos, if you look into doing the same, also need to make sure they correctly enable #![cfg_attr(docsrs, feature(doc_auto_cfg))] like this crate does (if appropriate).