typeck_root_def_id: improve doc comment

This was initially written to be exhaustive, but one more type that can only be type-checked with its containing item has since been added, and was not mentioned. So, make it future-proof by mentioning just the one example.

Also, a previous refactor left this less readable.

Author: tshepang

compiler/rustc_middle/src/ty/util.rs

@@ -642,12 +642,8 @@ impl<'tcx> TyCtxt<'tcx> {
     /// has its own type-checking context or "inference environment".
     ///
     /// For example, a closure has its own `DefId`, but it is type-checked
-    /// with the containing item. Similarly, an inline const block has its
-    /// own `DefId` but it is type-checked together with the containing item.
-    ///
-    /// Therefore, when we fetch the
-    /// `typeck` the closure, for example, we really wind up
-    /// fetching the `typeck` the enclosing fn item.
+    /// with the containing item. Therefore, when we fetch the `typeck` of the closure,
+    /// for example, we really wind up fetching the `typeck` of the enclosing fn item.
     pub fn typeck_root_def_id(self, def_id: DefId) -> DefId {
         let mut def_id = def_id;
         while self.is_typeck_child(def_id) {