Clarify niche optimization on enums with reprs

A repr on an enum with fields suppresses the optimization, but a repr on
a fieldless enum does not.

src/other-reprs.md

@@ -91,7 +91,7 @@ manipulate its tag and fields. See [the RFC][really-tagged] for details.
 
 These `repr`s have no effect on a struct.
 
-Adding an explicit `repr(u*)`, `repr(i*)`, or `repr(C)` to an enum suppresses the null-pointer optimization, like:
+Adding an explicit `repr(u*)`, `repr(i*)`, or `repr(C)` to an enum with fields suppresses the null-pointer optimization, like:
 
 ```rust
 # use std::mem::size_of;
@@ -110,6 +110,8 @@ assert_eq!(8, size_of::<MyOption<&u16>>());
 assert_eq!(16, size_of::<MyReprOption<&u16>>());
 ```
 
+This optimization still applies to fieldless enums with an explicit `repr(u*)`, `repr(i*)`, or `repr(C)`.
+
 ## repr(packed)
 
 `repr(packed)` forces Rust to strip any padding, and only align the type to a