Suppress warnings "module attribute @attr_name was set but never used"

Yes, consume the attributes or do not set them. This warning was introduced with Elixir 1.4 as far as I remember, so sldab had a lot of time to react on this change.

Elixir wise you can not do anything about the warning except for fixing it.

You might be able to create a macro api_doc with the generated: true option to declare the attribute?

I solved the problem.
It’s possible to use @before_compile hook and invoke Module.delete_attribute(env.module, :apidoc) inside it. So it removes all attributes before a compilation.
In my case, I use Phoenix controllers.
So I put in my_project_web.ex inside quote:

def controller
  quote do
    @before_compile MyProjectWeb
    # ...
  end
end

and in the root of MyProjectWeb module added function:

  def __before_compile__(env) do
    Module.delete_attribute(env.module, :apidoc)
  end