Update comments

ocaml/typing/ctype.mli

@@ -601,7 +601,8 @@ val check_type_externality : Env.t -> type_expr -> Jkind.Externality.t -> bool
    itself.
 
    2. Performs an upstream-compatibility check around immediacy if
-      [Language_extension.erasable_extensions_only ()] is [true].
+      [Language_extension.erasable_extensions_only ()] is [true]
+      and [upstream_compat] is [true].
 
    The check makes sure no generalized type variable can have jkind
    [immediate] or [immediate64]. An exception would be raised when

ocaml/typing/typecore.ml

@@ -8501,7 +8501,8 @@ and type_let ?check ?check_strict ?(force_toplevel = false)
           | Tpat_alias(_, id, _, _, _) -> Some id
           | _ -> None in
         (* We don't need to check upstream compatibility for expressions:
-           just ensuring that annotations are erasable is enough. *)
+           just ensuring none of the type parameters of the function are
+           immediate is enough. *)
         Ctype.check_and_update_generalized_ty_jkind
           ?name:pat_name ~loc:exp.exp_loc ~upstream_compat:false exp.exp_type
       in