Improve Documentation about component specification in rust-toolchain file

[jschwe]

Edit: I just saw that this will be possible once a new version is released

Describe the problem you are trying to solve

When building a project with a rust-toolchain file, rustup automatically fetches and installs the specified toolchain. However some projects might need additional components, e.g. in my case rust-src. Simply building the project fails until manual installation of the missing component.

Describe the solution you'd like

It would be nice to be able to specify additional components in the rust-toolchain file that are automatically installed when rustup fetches the specified toolchain. If the component is not available for the specified version, then rustup could print an error saying that the combination of toolchain and component specified in the rust-toolchain file do not exist.

[jschwe]

I discovered #2438, which solves exactly this, so I'm closing the issue.