Fix a few doc comment for compiler-interal API docs.

They only used `//` instead of `///` so weren't picked up by rustdoc.

Author: aDotInTheVoid

compiler/rustc_hir_typeck/src/typeck_root_ctxt.rs

@@ -18,7 +18,7 @@ use rustc_trait_selection::traits::{
 use std::cell::RefCell;
 use std::ops::Deref;
 
-// Data shared between a "typeck root" and its nested bodies,
+/// Data shared between a "typeck root" and its nested bodies,
 /// e.g. closures defined within the function. For example:
 /// ```ignore (illustrative)
 /// fn foo() {

compiler/rustc_mir_transform/src/elaborate_drops.rs

@@ -33,16 +33,16 @@ use std::fmt;
 /// as it would allow running a destructor on a place behind a reference:
 ///
 /// ```text
-// fn drop_term<T>(t: &mut T) {
-//     mir! {
-//         {
-//             Drop(*t, exit)
-//         }
-//         exit = {
-//             Return()
-//         }
-//     }
-// }
+/// fn drop_term<T>(t: &mut T) {
+///     mir! {
+///         {
+///             Drop(*t, exit)
+///         }
+///         exit = {
+///             Return()
+///         }
+///     }
+/// }
 /// ```
 pub struct ElaborateDrops;