Improve note for the invalid_reference_casting lint

Add link to the book interior mutability chapter,
https://doc.rust-lang.org/book/ch15-05-interior-mutability.html.

compiler/rustc_lint/messages.ftl

@@ -323,6 +323,8 @@ lint_invalid_reference_casting_assign_to_ref = assigning to `&T` is undefined be
 lint_invalid_reference_casting_borrow_as_mut = casting `&T` to `&mut T` is undefined behavior, even if the reference is unused, consider instead using an `UnsafeCell`
     .label = casting happend here
 
+lint_invalid_reference_casting_note_book = for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
+
 lint_lintpass_by_hand = implementing `LintPass` by hand
     .help = try using `declare_lint_pass!` or `impl_lint_pass!` instead
 

compiler/rustc_lint/src/lints.rs

@@ -764,11 +764,13 @@ pub enum InvalidFromUtf8Diag {
 #[derive(LintDiagnostic)]
 pub enum InvalidReferenceCastingDiag {
     #[diag(lint_invalid_reference_casting_borrow_as_mut)]
+    #[note(lint_invalid_reference_casting_note_book)]
     BorrowAsMut {
         #[label]
         orig_cast: Option<Span>,
     },
     #[diag(lint_invalid_reference_casting_assign_to_ref)]
+    #[note(lint_invalid_reference_casting_note_book)]
     AssignToRef {
         #[label]
         orig_cast: Option<Span>,