Suggestion: Support `textDocument/hover` for functions implementing a behaviour

**Provided example**

```elixir
defmodule MyModule do
  use GenServer

  @impl GenServer
  def init(arg) do
    # ...
  end
end
```

**Current behaviour**
Using hover on `init` only displays `MyModule.init`.

**Suggested behaviour**
Using hover on `init` additionally provides the documentation for the behaviour's `init` callback.