Document expression post-aggregators

[jacobtolar]

Description

There was no documentation about expression post-aggregators; adding that here.

Also adding a note that the arithmetic post-aggregator always uses floating point arithmetic ( per the latest commit.)

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This PR has:

• been self-reviewed.
  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• added documentation for new or modified features or behaviors.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.
• been tested in a test Druid cluster. (I didn't test exhaustively, but I verified that some queries with the syntax documented here work as expected).

[FrankChen021]

Thanks for your contribution. Please fix the typo above.

[stale]

This pull request has been marked as stale due to 60 days of inactivity. It will be closed in 4 weeks if no further activity occurs. If you think that's incorrect or this pull request should instead be reviewed, please simply write any comment. Even if closed, you can still revive the PR at any time or discuss it on the dev@druid.apache.org list. Thank you for your contributions.

[stale]

This pull request/issue is no longer marked as stale.