Document (and perhaps change) interaction between `rust-toolchain.toml` and other override sources for components and targets

Problem you are trying to solve

It occurred to me that, even though I am using stable Rust, I could use a rust-toolchain.toml file to declare the targets and components my project requires, making it more convenient to use. So, I did this, and also replaced the part of my CI configuration which ran explicit rustup commands with rustup override set "${{ matrix.toolchain }}".

This then failed, revealing to me that rustup override set appears to cause the rust-toolchain.toml file to be completely ignored, even though the file can specify targets and components which rustup override set cannot.

Solution you'd like

1. Explicitly document the (non-)inheritance behavior at https://rust-lang.github.io/rustup/overrides.html.
2. If feasible, add a means to request “override the toolchain but install the same targets and components as would otherwise have been installed" (or make that the default behavior if stability policy allows).

Notes

The workaround I've found to get the effect I wanted is

sed -i "s/stable/${{ matrix.toolchain }}/" rust-toolchain.toml
rustup show

which isn't elegant and leaves the working tree modified.