Add `docsrs` cfg as a well known `--check-cfg`

Now that rust-lang/docs.rs#2390 has been merged we can add the `docsrs` cfg in Cargo well known --check-cfg "list". The `docsrs` cfg used by at least [3k project on GitHub](https://github.com/search?q=lang%3Atoml+%2Frustdoc-args+%3D+%5C%5B%22--cfg%22%2C+%22docsrs%22%5C%5D%2F+NOT+is%3Afork&type=code&repo=&langOverride=&start_value=1) alone; including the cfg will help reduce the impact of enabling by default this feature.

> We include it here (in Cargo) instead of rustc, since there is a much closer relationship between Cargo and docs.rs than rustc and docs.rs. In particular, all users of docs.rs use Cargo, but not all users of rustc (like Rust-for-Linux) use docs.rs.

This is part of the last remaining bits of the `--check-cfg` feature.

r? `@epage`

- Don't instruct docs.rs to set it, it does so by default now:
  rust-lang/docs.rs#2390
- Because of that, check-cfg also recognizes it and does not need
  extra configuration either.

- Don't instruct docs.rs to set it, it does so by default now:
  rust-lang/docs.rs#2390
- Because of that, check-cfg also recognizes it and does not need
  extra configuration either.

Don't need to tell docs.rs to enable `docsrs` cfg. It does it automatically as of rust-lang/docs.rs#2390 (comment)

While this change isn't in our MSRV yet, we were only using this when building for docs.rs, where we use the latest anyway.

See merge request tpo/core/arti!2308

The flag is passed automatically for all docs.rs builds. See:
rust-lang/docs.rs#2390

- Don't instruct docs.rs to set it, it does so by default now:
  rust-lang/docs.rs#2390
- Because of that, check-cfg also recognizes it and does not need
  extra configuration either.

See rust-lang/docs.rs#2390 (note that the PR
title is misleading; `--cfg docsrs` is only added for the "terminal
crate")

…-random#565)

The flag is passed automatically for all docs.rs builds. See:
rust-lang/docs.rs#2390