Better document what exactly the action does

[RalfJung]

The summary says that this "installs a Rust toolchain using rustup". However it clearly does more than that, since the toolchain also becomes the default toolchain to use afterwards. How exactly is that done? Is it a rustup default or a rustup override set or something else? The details matter when there are rust-toolchain files in the folder, so it would be great to have this documented.

[szepeviktor]

https://github.com/dtolnay/rust-toolchain/blob/f361669954a8ecfc00a3443f35f9ac8e610ffc06/action.yml#L75