Allow specifying rustup doc toolchain

[pickfire]

As discussed in rust-lang/rust#53158. If we would like to open the nightly docs with stable as default toolchain. We need to:

$ rustup default nightly
$ rustup doc --std
$ rustup default stable

As well, part of my confusions is that I thought the following works.

• rustup +nightly doc (cargo uses this so I thought it applies to rustup)
• rustup doc --toolchain=nightly (rustup component add clippy preview --toolchain=nightly works though)

Or maybe there's a quick way to switch between stable and nightly docs if both toolchains are installed? Answered in rust-lang/rust#53158 (comment) by @cuviper