Add better documentation for timestamp prop

[TamiTakamiya]

Timestamps attached to messages keep changing.

Screenshare.-.2024-11-06.8_29_24.AM.mp4

Please look at the timestamp attachd to the message "Hello" in this recording.
Initially it is displayed as 8:28:14 AM. As the conversation progresses, it is changed to 8:29:18 AM and then to 8:29:23 AM.

Those timestamps should stay in fixed values.

[rebeccaalpert]

I talked about this with Tami on Slack. We thankfully have a timestamp prop that can be used for persistent timestamps (thanks Rohit and team for adding it before I started). I think we should add an explicit example that calls this out. We have one setting a relative timestamp, but we could be more explicit.

[rebeccaalpert]

Recategorizing as docs since we'll use this for examples.