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>