PARQUET-2139: Clarify ColumnChunk::file_offset is not used

[alamb]

See also mailint list discussion: https://lists.apache.org/thread/r6r2cvzrdoorq6h6gqwh0b1hbfbhxv29

This field is not written correctly in the Java implementation and it not read in other implementations (like rust). Let's clarify this in the format docs

Jira/GH

Related to https://github.com/apache/parquet-java/issues/2678

• [ ] My PR addresses https://issues.apache.org/jira/browse/PARQUET-2139

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[alamb]

Superseded by https://github.com/apache/parquet-format/pull/440