@:dox(hide) works only if applied to every platform

Currently, using the `@:dox(hide)` field works only if every platform is compiled with this meta-data.

It would be great if `@:dox(hide)` or another meta-field allowed support for logically flagging a field as unavailable.

For example, the following would compile as "Available for all platforms":

```haxe
public static function encode( s : String ) : String {
throw "Not implemented";
}
```

The goal is to be able to support something such as:

```haxe
#if (!cpp && !neko) @:dox(hide) #end
public static function encode( s : String ) : String {
throw "Not implemented";
}
```

To get "Available on cpp,neko"