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",