docs: rust: Clarify that 'rustup override' applies to build directory

'rustup override' is required to be set for the build directory and not
necessarily the kernel source tree (unless the build directory is its
subdir).

Clarify the same in the Quick Start guide.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Originally-pointed-out-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://github.com/Rust-for-Linux/linux/commit/f2238e7
Link: https://lore.kernel.org/r/e2b943eca92abebbf035447b3569f09a7176c770.1702366951.git.viresh.kumar@linaro.org


[ Reworded and fixed quotes for `--path` and `set`. ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>