MetaInfo.to_string: document @:enum abstract meta instead of internal one

It seems like this meta is used for two different things - since one of them is internal and not displayed on --help-metas, it makes more sense to document the one that's public API instead.
The description is from http://haxe.org/manual/cr-metadata.html.

common.ml

@@ -387,7 +387,7 @@ module MetaInfo = struct
 		| Deprecated -> ":deprecated",("Automatically added by -java-lib on class fields annotated with @Deprecated annotation. Has no effect on types compiled by Haxe",[Platform Java; UsedOnEither [TClass;TEnum;TClassField]])
 		| DirectlyUsed -> ":directlyUsed",("Marks types that are directly referenced by non-extern code",[Internal])
 		| DynamicObject -> ":dynamicObject",("Used internally to identify the Dynamic Object implementation",[Platforms [Java;Cs]; UsedOn TClass; Internal])
-		| Enum -> ":enum",("Used internally to annotate a class that was generated from an enum",[Platforms [Java;Cs]; UsedOn TClass; Internal])
+		| Enum -> ":enum",("Defines finite value sets to abstract definitions",[UsedOn TAbstract])
 		| EnumConstructorParam -> ":enumConstructorParam",("Used internally to annotate GADT type parameters",[UsedOn TClass; Internal])
 		| Event -> ":event",("Automatically added by -net-lib on events. Has no effect on types compiled by Haxe",[Platform Cs; UsedOn TClassField])
 		| Exhaustive -> ":exhaustive",("",[Internal])