add note about problems with spin_until_future_complete (#2849)

* add note indicating that spin_until_future_complete should be used only with futures generated by ROS entities

Signed-off-by: Alberto Soragna <alberto.soragna@gmail.com>

* fix indentation and improve explanation of consequences

Signed-off-by: Alberto Soragna <alberto.soragna@gmail.com>

---------

Signed-off-by: Alberto Soragna <alberto.soragna@gmail.com>

Author: alsora

rclcpp/include/rclcpp/executor.hpp

@@ -371,6 +371,9 @@ class Executor
    *   If the time spent inside the blocking loop exceeds this timeout, return a TIMEOUT return
    *   code.
    * \return The return code, one of `SUCCESS`, `INTERRUPTED`, or `TIMEOUT`.
+   * \note This method will check the future and the timeout only when the executor is woken up.
+   *   If this future is unrelated to an executor's entity, this method will not correctly detect
+   *   when it's completed and therefore may wait forever and never time out.
    */
   template<typename FutureT, typename TimeRepT = int64_t, typename TimeT = std::milli>
   FutureReturnCode