Hide report_cb/1 from docs

[wojtekmach]

for posterity, with this code:

-module(foo).
-export([f1/0]).

%% @hidden
f1() -> ok.

we end up with the following chunk:

{docs_v1,[{file,"foo.erl"},{location,1}],
         erlang,<<"application/erlang+html">>,none,#{},
         [{{function,f1,0},
           [{file,"foo.erl"},{location,5}],
           [<<"f1()">>],
           none,#{}}]}

note the function gets the doc none.

when we switch @hidden to @private:

{docs_v1,[{file,"foo.erl"},{location,1}],
         erlang,<<"application/erlang+html">>,none,#{},
         [{{function,f1,0},
           [{file,"foo.erl"},{location,5}],
           [<<"f1()">>],
           hidden,#{}}]}

we get what we want, hidden.

[wojtekmach]

we're gonna tackle this on ExDoc side.