Add documentation retrieval for module's built-in attributes

[Goose97]

Closes #227

[lukaszsamson]

That's a good first step @Goose97 but 2 more things should be covered here.

1. we need docs on attribute completion as well
2. I was thinking of extending and using BuiltinAttributes module here. It has some more documented elixir attributes that are not returned by Module.reserved_attributes. We need to copy docs for those from elixir repo

[Goose97]

I've adjusted the PR following your suggestions.

I removed struct from the buil-in list, because:

1. It was renamed to __struct__
2. This attribute is an implementation detail, so would it make more sense it we don't expose it?

[lukaszsamson]

Thanks