Catching @doc deprecated: "..." annotations

I am trying to hook into the @doc deprecated: "message" annotation using __on_definition__.

defmodule ExampleCompiler do
  def __on_definition__(env, _, fun, _, _, _) do
    doc = Module.get_attribute(env.module, :doc)
    IO.inspect(doc, label: fun)
  end
end

defmodule DemoModule do
  @on_definition ExampleCompiler

  @doc "hello"
  def hello() do
  end

  @doc deprecated: "hello2"
  def hello2() do
  end
end

Output of compilation:

hello: {11, "hello"}
hello2: nil

So, for some reason in the on_definition hook of the hello2 function, the module @doc attribute is empty whereas a keyword list [deprecated: ".."] would be expected. There seems to be something special in the compiler regarding the @doc module attribute, because when I change it to @doc2 it works as expected.

Also note the line number annotation in the first definition, this is also inconsistent with other module attributes. I have already been loking in the Module docs but I don’t see anything that tells me that @doc is special. Help appreciated