ORC-1671: update timestamp doc in specfication

[Jefffrey]

What changes were proposed in this pull request?

Updating timestamp specification documentation to be more accurate to the implementation.

Why are the changes needed?

Timestamp specification is not clear on some nuances of it's implementation. Pulling knowledge from the following for these updates:

• https://github.com/apache/arrow/issues/34590
• https://issues.apache.org/jira/browse/ORC-10
• https://issues.apache.org/jira/browse/ORC-784
• https://issues.apache.org/jira/browse/ORC-763
• https://issues.apache.org/jira/browse/ORC-189
• C++ implementation

How was this patch tested?

Was this patch authored or co-authored using generative AI tooling?

No