Rust version policy: add footnote to clarify stability

Now that the "Unstable features" page includes an explanation
of the stability of the Rust language, explicitly mention it
in this page too, linking (again) that other page in case the
reader wants more details.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

src/Rust-version-policy.md

@@ -6,10 +6,12 @@ The kernel documents the [minimal requirements](https://docs.kernel.org/process/
 
 > A particular version of the Rust toolchain is required. Newer versions may or may not work because the kernel depends on some unstable Rust features, for the moment.
 
-The reason is that we cannot guarantee newer Rust versions will work due to the [unstable features](Unstable-features.md) in use. Removing the need for them is a priority in order to be able to eventually declare a minimum Rust version for the kernel.
+The reason is that we cannot guarantee newer Rust versions will work due to the [unstable features](Unstable-features.md) in use[^rust-is-stable]. Removing the need for them is a priority in order to be able to eventually declare a minimum Rust version for the kernel.
 
 Having said that, generally speaking, newer versions should work, as long as one patches any potential compilation errors coming from changes in unstable features.
 
+[^rust-is-stable]: Note that the Rust language is stable, i.e. it promises backwards compatibility. See the [Unstable features](Unstable-features.md) page for details.
+
 ### Distribution toolchains
 
 Some Linux distributions provide Rust toolchains (i.e. built by the distribution maintainers, rather than redistributing the ones from https://www.rust-lang.org). These toolchains should be fine to use, as long as they have not been modified in unexpected ways (and keeping in mind the versioning limitations).