The source for Rust's 'core' library is needed to build the kernel with
Rust support. This sometimes needs to be obtained by hand when using a
standalone version of 'rustc' not managed by 'rustup'. Currently, the
documentation suggests cloning the 'rust' repository to obtain these
sources, but this is quite slow (on the order of a multiple minutes).

Change this documentation to suggest using the source tarball instead.
The tarball includes only needed files (<5M) and is significantly faster
to download; this is more in line with what 'rustup' does.

Also simplify wording of the relevant section.

Link: https://github.com/Rust-for-Linux/linux/pull/1024Signed-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/20230803060437.12157-2-tmgross@umich.eduSigned-off-by: Miguel Ojeda <ojeda@kernel.org>