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>

Documentation/rust/quick-start.rst

@@ -33,14 +33,18 @@ A particular version of the Rust compiler is required. Newer versions may or
 may not work because, for the moment, the kernel depends on some unstable
 Rust features.
 
-If ``rustup`` is being used, enter the checked out source code directory
-and run::
+If ``rustup`` is being used, enter the kernel build directory (or use
+``--path=<build-dir>`` argument to the ``set`` sub-command) and run::
 
 	rustup override set $(scripts/min-tool-version.sh rustc)
 
 This will configure your working directory to use the correct version of
-``rustc`` without affecting your default toolchain. If you are not using
-``rustup``, fetch a standalone installer from:
+``rustc`` without affecting your default toolchain.
+
+Note that the override applies to the current working directory (and its
+sub-directories).
+
+If you are not using ``rustup``, fetch a standalone installer from:
 
 	https://forge.rust-lang.org/infra/other-installation-methods.html#standalone