Document how recursion is handled for ty::Ty

Based on this forum discussion:
https://internals.rust-lang.org/t/recursive-type-representation-in-rustc/15235/4

compiler/rustc_middle/src/ty/mod.rs

@@ -1700,7 +1700,7 @@ impl ReprOptions {
 
 impl<'tcx> FieldDef {
     /// Returns the type of this field. The resulting type is not normalized. The `subst` is
-    /// typically obtained via the second field of `TyKind::AdtDef`.
+    /// typically obtained via the second field of [`TyKind::Adt`].
     pub fn ty(&self, tcx: TyCtxt<'tcx>, subst: SubstsRef<'tcx>) -> Ty<'tcx> {
         tcx.type_of(self.did).subst(tcx, subst)
     }