rust: build_error: improve documentation

This is part of the effort to minimize the differences of the `rust`
branch with respect to mainline in order to eventually drop it.

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

Author: ojeda

rust/build_error.rs

@@ -2,19 +2,26 @@
 
 //! Build-time error.
 //!
-//! This crate provides a function `build_error`, which will panic in
-//! compile-time if executed in const context, and will cause a build error
-//! if not executed at compile time and the optimizer does not optimise away the
-//! call.
+//! This crate provides a [const function][const-functions] `build_error`, which will panic in
+//! compile-time if executed in [const context][const-context], and will cause a build error
+//! if not executed at compile time and the optimizer does not optimise away the call.
 //!
 //! It is used by `build_assert!` in the kernel crate, allowing checking of
 //! conditions that could be checked statically, but could not be enforced in
-//! Rust yet (e.g. perform some checks in const functions, but those
+//! Rust yet (e.g. perform some checks in [const functions][const-functions], but those
 //! functions could still be called in the runtime).
+//!
+//! For details on constant evaluation in Rust, please see the [Reference][const-eval].
+//!
+//! [const-eval]: https://doc.rust-lang.org/reference/const_eval.html
+//! [const-functions]: https://doc.rust-lang.org/reference/const_eval.html#const-functions
+//! [const-context]: https://doc.rust-lang.org/reference/const_eval.html#const-context
 
 #![no_std]
 
-/// Panics if executed in const context, or triggers a build error if not.
+/// Panics if executed in [const context][const-context], or triggers a build error if not.
+///
+/// [const-context]: https://doc.rust-lang.org/reference/const_eval.html#const-context
 #[inline(never)]
 #[cold]
 #[export_name = "rust_build_error"]