Add doc for broadcast.

src/ngraph/op.hpp

@@ -101,6 +101,12 @@ namespace ngraph
     class BroadcastCall : public BuiltinCall
     {
     public:
+        /**
+         ** /param arg The tensor view to be broadcast.
+         ** /param shape The shape of the result
+         ** /param broadcast_axes The axis positions (0-based) in the result that are being broadcast.
+         **  the remaining axes in shape must be the same as the shape of arg.
+         **/
         BroadcastCall(const Node::ptr& arg, const Shape& shape, std::vector<size_t> broadcast_axes)
             : BuiltinCall(s_op, {arg})
             , m_shape(shape)