Method on_document_begin

test/basic_parser.cpp:1685–1685

            constexpr static std::size_t max_string_size = std::size_t(-1);

            bool on_document_begin( std::error_code& ) { return true; }
            bool on_document_end( std::error_code& ) { return true; }
            bool on_object_begin( std::error_code& ) { return true; }
            bool on_object_end( std::size_t, std::error_code& ) { return true; }