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 apache/parquet-java#2678

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

Commits

• My commits all reference Jira issues in their subject lines. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

[alamb]

Superseded by #440