reference.md: clarify the limitation of type alias on an enum

Cf. rust-lang/rust#26264
Signed-off-by: NNODA, Kai <nodakai@gmail.com>

src/doc/reference.md

@@ -1118,6 +1118,16 @@ type Point = (u8, u8);
 let p: Point = (41, 68);
 ```
 
+Currently a type alias to an enum type cannot be used to qualify the
+constructors:
+
+```
+enum E { A }
+type F = E;
+let _: F = E::A;  // OK
+// let _: F = F::A;  // Doesn't work
+```
+
 ### Structs
 
 A _struct_ is a nominal [struct type](#struct-types) defined with the