Clarify that rcl_take_response() populates request_header (ros2#205)

* Document rcl_take_response() populates the header

rcl/include/rcl/client.h

@@ -257,6 +257,8 @@ rcl_send_request(const rcl_client_t * client, const void * ros_request, int64_t
  * occur.
  * The request_header is an rmw struct for meta-information about the request
  * sent (e.g. the sequence number).
+ * The caller must provide a pointer to an allocated struct.
+ * This function will populate the struct's fields.
  * `ros_response` should point to an already allocated ROS response message
  * struct of the correct type, into which the response from the service will be
  * copied.