PARQUET-2435: Clarify behavior of DELTA_BINARY_PACKED encoding

[etseidl]

Provide some guidance around the issue of how many bits may be used when encoding DELTA_BINARY_PACKED data.

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[etseidl]

Thank you for the comments @wgtmac @pitrou @mapleFU. I've added the prohibition language. If there is consensus on forbidding the use of extra bits, then I can remove the long paragraph.

[tustvold]

I am likely missing some context here, but I would agree with @pitrou that an encoder producing data with more bits than the physical type is a bug in the encoder, and not to mention sub-optimal

[pitrou]

The latest proposed changes look fine to me. I'll let others chime in before potentially merging.