Add info to duration message and time message comments (backport #176)

[mergify]

Closes #175

Makes comments for duration messages and time messages more clear, and adds examples for the message types.

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This is an automatic backport of pull request #176 done by Mergify.

[mergify]

Cherry-pick of 355006b has failed:

On branch mergify/bp/humble/pr-176
Your branch is up to date with 'origin/humble'.

You are currently cherry-picking commit 355006b.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   builtin_interfaces/msg/Duration.msg
	both modified:   builtin_interfaces/msg/Time.msg

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[ahcorde]

Pulls: #179
Gist: https://gist.githubusercontent.com/ahcorde/4270b852b7cbc5cf3c2290f4074e4aa1/raw/6aec71facb6dc6a76f52c0c9675ade1220e89ab9/ros2.repos
BUILD args: --packages-above-and-dependencies rcl_interfaces
TEST args: --packages-above rcl_interfaces
ROS Distro: humble
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/15944

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