[MACRO] Allow macros to modify class documentation

In my attempt to develop a macro which injects documentation into indicated classes and fields, I have not found a means that the former is currently possible.

I tried the obvious, assigning directly inside a build macro:

```haxe
var cls:ClassType = Context.getLocalClass().get();
cls.doc = "Hello, world!"
```

This does not function as I would hope (it works great for fields though). The assignment has no effect since I am modifying the response from `Ref.get()`, not the class itself.

When using an initialization macro, `onAfterGenerate` and `onAfterInitMacros` have no access to the classes, while `onGenerate` and `onAfterTyping` only provide `Ref`s to the classes. MetadataAccess is provided to inject metadata at this stage, and build macros can be used to modify fields of those types completely, but I have found no means to inject documentation onto the class itself.

Either a dedicated means of injecting class documentation, a built-in compiler metadata which takes documentation to apply as an argument, or a workaround to this issue that I'm overlooking, any of these would be greatly appreciated.