Minor corection to enum traits. (#4393)

Author: Crell

language/enumerations.xml

@@ -511,7 +511,7 @@ enum Size
 
   <para>Enumerations may leverage traits, which will behave the same as on classes.
    The caveat is that traits <literal>use</literal>d in an enum must not contain properties.
-   They may only include methods and static methods.  A trait with properties will
+   They may only include methods, static methods, and constants.  A trait with properties will
    result in a fatal error.
   </para>