Document tf2::TimePoint vs rclcpp::Time (ros2#3079) (ros2#3081)

* Document tf2::TimePoint vs rclcpp::Time

Signed-off-by: Shane Loretz <sloretz@osrfoundation.org>
(cherry picked from commit 3d8b551)

Co-authored-by: Shane Loretz <sloretz@osrfoundation.org>

source/Tutorials/Intermediate/Tf2/Learning-About-Tf2-And-Time-Cpp.rst

@@ -44,6 +44,14 @@ Open ``turtle_tf2_listener.cpp`` and take a look at the ``lookupTransform()`` ca
       tf2::TimePointZero);
 
 You can see that we specified a time equal to 0 by calling ``tf2::TimePointZero``.
+
+.. note::
+
+    The ``tf2`` package has it's own time type ``tf2::TimePoint``, which is different from ``rclcpp::Time``.
+    Many APIs in the package ``tf2_ros`` automatically convert between ``rclcpp::Time`` and ``tf2::TimePoint``.
+
+    ``rclcpp::Time(0, 0, this->get_clock()->get_clock_type())`` could have been used here, but it would have been converted to ``tf2::TimePointZero`` anyways.
+
 For tf2, time 0 means "the latest available" transform in the buffer.
 Now, change this line to get the transform at the current time, ``this->get_clock()->now()``: