release note about change to log file flushing (#3193)

* release note about change to log file flushing

ros2/rcl_logging#95

* mention the volatility of the env var

* Apply suggestions from code review

Co-authored-by: Ivan Santiago Paunovic <ivanpauno@ekumenlabs.com>
Co-authored-by: Audrow Nash <audrow@openrobotics.org>

* emphasize note about experimental status

also link to issue about logging config file plans

Co-authored-by: Ivan Santiago Paunovic <ivanpauno@ekumenlabs.com>
Co-authored-by: Audrow Nash <audrow@openrobotics.org>

source/Releases/Release-Iron-Irwini.rst

@@ -68,6 +68,23 @@ Classes that were changed:
 
 The old class names are still there, but will be deprecated.
 
+Change to the default console logging file flushing behavior
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+This specifically applies to the default ``spdlog`` based logging backend in ROS 2, implemented in ``rcl_logging_spdlog``.
+Log file flushing was changed to flush every time an "error" log message is used, e.g. each `RCLCPP_ERROR()` call, and also periodically every five seconds.
+
+Previously, ``spdlog`` was used without configuring anything other than creating the sink for logging to a file.
+
+We tested the change and did not find that the CPU overhead was significant, even on machines with slow disks (e.g. sd cards).
+However, if this change is causing you problems, you can get the old behavior by setting the ``RCL_LOGGING_SPDLOG_EXPERIMENTAL_OLD_FLUSHING_BEHAVIOR=1`` environment variable.
+
+Later we would like to have support for a full configuration file (see: https://github.com/ros2/rcl_logging/issues/92), giving you more flexibility in how the logging is done, but that is work that is only planned right now.
+
+  Therefore, **this environment variable should be considered experimental and subject to removal without deprecation in the future**, when we add config file support for the ``rcl_logging_spdlog`` logging backend.
+
+See this pull request for more details about the change: https://github.com/ros2/rcl_logging/pull/95
+
 Known Issues
 ------------