This is a follow up to the Aggregate Foxy Message API Review

From: https://github.com/ros2/common_interfaces/pull/86/files#r393329045

Improve documentation in the message to indicate that it's specifically a ROS time with time since epoch, or time since the start of simulation.

We could consider making a separate Steady time type that can support things like hardware timestamps. Potentially providing a UUID for identifying the epoch so that only times on the same epoch can be compared.