IEx.Helpers.b should show callbacks even there are no docs

[josevalim]

Here are two failing test cases:

iex(5)> b Enumerable
No callbacks for Enumerable were found
iex(6)> Enumerable.behaviour_info :callbacks
[slice: 1, reduce: 3, member?: 2, count: 1]
iex(7)> b :gen_server
:gen_server was not compiled with docs
[init: 1, ...]

I suspect this is happening because we are assuming the docs chunk to be the source of truth. But the docs chunk should only be used to attach documentation (or skip the callback in case it is :hidden).

[fertapric]

I'm working on this one :)

[josevalim]

Closing in favor of #8514.