Document timezone semantics

### Is your feature request related to a problem or challenge?

The automatic coercion and comparison rules for timestamps with timezones (especially how timestamps with `None`) are treated is not clear -- for example https://github.com/apache/datafusion/discussions/10311 from @Abdullahsab3

### Describe the solution you'd like

I would like something in the

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

Start by reviewing the timestamp timezone coercion and comparison behavior described in the issue and the linked DataFusion discussion 10311. Document how timestamps with timezones, including timestamps with None, are coerced and compared; the issue does not identify a documentation file or test to update.

Written by the indexing model from the issue text.