Document INT96 timestamps

[asfimport]

Although considered as deprecated, they should be documented as the format is quite special.

Reporter: Uwe Korn / @xhochy Assignee: Uwe Korn / @xhochy

PRs and other links:

• GitHub Pull Request #49

_{Note: This issue was originally created as PARQUET-861. Please see the migration documentation for further details.}

[asfimport]

Uwe Korn / @xhochy: PR: https://github.com/apache/parquet-format/pull/49