docs: rust: clarify what 'rustup override' does

The behavior of 'rustup override' is not very well known. Add a small
note about what it does, so users have a better understanding of how it
affects their system toolchain (i.e., it does not affect system
toolchain and only sets a directory-specific override).

Signed-off-by: Trevor Gross <tmgross@umich.edu>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20230803060437.12157-3-tmgross@umich.edu
[ Undid the `:` to `::` change. ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: tgross35

Documentation/rust/quick-start.rst

@@ -38,7 +38,9 @@ and run::
 
 	rustup override set $(scripts/min-tool-version.sh rustc)
 
-Otherwise, fetch a standalone installer from:
+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:
 
 	https://forge.rust-lang.org/infra/other-installation-methods.html#standalone