Add suggestion to use `rustup doc`

[metaspace]

Suggest to use rustup doc std::fmt to open the documentation for std::fmt.

[google-cla[bot]]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[mgeisler]

@metaspace, could you sign the CLA so we can merge this?

[metaspace]

@metaspace, could you sign the CLA so we can merge this?

Done! Had to check with employer how to handle CLA first.

[djmitche]

Thanks!