Added an "Explanation" header and expanded that section for the newly added lint.

Author: pnkfelix

compiler/rustc_lint_defs/src/builtin.rs

@@ -2819,10 +2819,18 @@ declare_lint! {
  /// }
  /// ```
  ///
+ /// ### Explanation
+ ///
+ /// In the 1.77 release, the const evaluation machinery adopted some
+ /// stricter rules to reject expressions with values that could
+ /// end up holding mutable references to state stored in static memory
+ /// (which is inherently immutable).
+ ///
  /// This is a [future-incompatible] lint to ease the transition to an error.
  /// See [issue #122153] for more details.
  ///
  /// [issue #122153]: https://github.com/rust-lang/rust/issues/122153
+ /// [future-incompatible]: ../index.md#future-incompatible-lints
  pub CONST_EVAL_MUTABLE_PTR_IN_FINAL_VALUE,
  Warn,
  "detects a mutable pointer that has leaked into final value of a const expression",