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 <[email protected]>
Reviewed-by: Trevor Gross <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[ Avoided breaking links in two lines. - Miguel ]
Signed-off-by: Miguel Ojeda <[email protected]>

Author: dirkbehme

Documentation/rust/general-information.rst

@@ -7,6 +7,14 @@ This document contains useful information to know when working with
 the Rust support in the kernel.
 
 
+``no_std``
+----------
+
+The Rust support in the kernel can link only `core <https://doc.rust-lang.org/core/>`_,
+but not `std <https://doc.rust-lang.org/std/>`_. Crates for use in the
+kernel must opt into this behavior using the ``#![no_std]`` attribute.
+
+
 Code documentation
 ------------------