docs: rust: no_std is used

Using the #![no_std] attribute in the Rust kernel support is different
to the default Rust usage. Mention this in the Documentation.

Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Link: https://lore.kernel.org/r/20240610122332.3858571-1-dirk.behme@de.bosch.com


[ Avoided breaking links in two lines. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>