Unstable features: clarify that the Rust language is stable

A LWN discussion [1] seemed to indicate that it is unclear that
the Rust language is stable.

Thus clarify further this point, including a couple links to the
stability promise in Rust 1.0 as well as The Rust Edition Guide.

Link: https://lwn.net/Articles/944201/ [1]
Signed-off-by: Miguel Ojeda <[email protected]>

Author: ojeda

src/Unstable-features.md

@@ -2,7 +2,9 @@
 
 ## Introduction
 
-The kernel uses some Rust unstable features. These features can only be accessed by opting into them. They are typically used as a way to introduce new features into the language, library and toolchain to allow end users to experiment with them and provide feedback before committing to them.
+The Rust language is stable, i.e. it promises [backwards compatibility](https://blog.rust-lang.org/2014/10/30/Stability.html) within the same [edition](https://doc.rust-lang.org/edition-guide/editions/), with a few exceptions, such as reserving the right to patch safety holes. The kernel currently uses Edition 2021, which is the latest.
+
+On top of that, the kernel uses some Rust unstable features. These features can only be accessed by opting into them. They are typically used as a way to introduce new features into the language, library and toolchain to allow end users to experiment with them and provide feedback before committing to them.
 
 "Unstable" in this context means the feature may change in future versions, i.e. backwards compatibility is not promised for those features. It does not necessarily imply that the features are broken.