Improve documentation for preserveInstants option use.

When using preserveInstants, because of using the DATETIME datatype that must normally not be used when dealing with different timezone, this is a workaround for compatibility.

Current documentation states that preserveInstants is 'not recommended for long-term solutions'. That is a wrong statement: it's not a wrong 'long term' solution. The problem is that this approach has inherent limitations rather than being merely a temporary solution.

The real issue is that it can produce incorrect results when working with dates far from the current time period, such as 9999-12-31, due to how timezone rules and daylight saving time transitions are handled across different historical periods.

The goal of this task is to document the problem that might occurs, as this might not be problematic at all depending on use case.