Should have discoverable documentation on tf2::TimePointZero vs. other constructs

[EricCousineau-TRI]

From #892 (comment)

• https://github.com/ros2/rviz/tree/humble#pluggable-transformation-library - describes pluggable transforms, thus may merit usage of rclcpp::Time(0, 0, clock->get_clock_type())
• https://github.com/ros2/rviz/blob/humble/docs/plugin_development.md - describes plugin development, mentions using TransfomerGuard, but provides no explicit guidance on when to use tf2::TImePointZero vs. rclcpp::Time(0, 0, ...).
• https://docs.ros.org/en/humble/Tutorials/Intermediate/Tf2/Learning-About-Tf2-And-Time-Cpp.html#tf2-and-time - mentions tf2::TimePointZero, but makes no mention of when to prefer other construct
• When to use rclcpp::TimePoint(0, 0) vs. rclcpp::TimePoint(0, 0, clock->get_clock_type()) unclear, will probably admit bugs

It would be nice to have discoverable documentation for this in some location.

\cc @sloretz