Allow @:inheritDoc var field to search for documentation in implemented interfaces

[RblSb]

This PR allows @:inheritDoc (without arguments) to search parent interface field for documentation, if there is no parent class field doc.

[kLabz]

Isn't that use case pretty niche? A build macro (or addGlobalMetadata) could do that job easily

[RblSb]

I guess it can work, then there is only @:inheritDoc improvement to also check interfaces field and not only parent class field, should be useful

[kLabz]

I think this makes sense. A test would be nice, or at least a code example of what case it helps with (I could then help with writing the test if needed)

[RblSb]

@kLabz this should be ok

[kLabz]

I'm not sure. What are the rules here for deciding what documentation is included or not?

[RblSb]

class Foo implements IFoo implements IFoo2 extends Parent {
	/** Foo test **/
	@:inheritDoc public override function test():Void {}
}

It concats field documentation from first non-empty parent documentation to function test(), always checking parent class test() first and then all interfaces in declaration order (Parent.test then IFoo.test then IFoo2.test docs).