List behaviour callbacks in Edoc when using -callback attribute

Defining a behaviour_info/1 function is no longer the only way to
define a behaviour; you can use -callback attributes as well.  This
change makes edoc aware of the latter, such that the module
documentation page for a behaviour module will list the name and arity
of the required callback functions.

Ideally, edoc should use the type information present in the callback
attributes, but with this change, the documentation is at least no
worse than when using an explicit behaviour_info function.

src/edoc_data.erl

@@ -167,7 +167,10 @@ callbacks(Es, Module, Env, Opts) ->
     case lists:any(fun (#entry{name = {behaviour_info, 1}}) -> true;
 		       (_) -> false
 		   end,
-		   Es) of
+		   Es)
+	orelse
+	lists:keymember(callback, 1, Module#module.attributes)
+    of
 	true ->
 	    try (Module#module.name):behaviour_info(callbacks) of
 		Fs ->