[docs] add note about callback lifetime for {on, post}_set_parameter_callback (#1981)
[docs] add note about callback lifetime for {on, post}_set_parameter_callback (#1981)

[docs] add note about {on, post, pre}_set_parameter_callback lifetime

Signed-off-by: Brian Chen <0488230585990a2d3bbca49694fb1318e9ababcf@openrobotics.org>
ros2/rclcpp,ros2/rclcpp