Add timestamp_field to the documentation

[ApolloLV]

Description

I noticed the documentation references timestamp_field in several places, but never actually documents the field itself.
I added a short description of what it does to the documentation.

Checklist

• I have reviewed the contributing guidelines.
• I have included unit tests for my changes or additions.
• I have successfully run make test-docker with my changes.
• I have manually tested all relevant modes of the change in this PR.
• I have updated the documentation.
• I have updated the changelog.

Questions or Comments

As this changes no actual source code, I have skipped the tests.
I'm not sure if we need to mention this pure documentation change in the changelog?

[jertel]

Thanks for taking the time to add this missing piece. Yes, please update the CHANGELOG.md. I prefer to over communicate, even if it's a small doc change.

[ApolloLV]

I've added it to the changelog and merged in the current master. Should be good to go now.