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ARROW-412: [Format][Documentation] Clarify that Buffer.size in Flatbuffers should reflect the actual memory size rather than the padded size

There had previously been some questions about whether the `Buffer` Flatbuffers struct should include padding bytes in its `length` or not. This adds documentation that the "length" need not be prescriptive about this -- if the writer intends the reader to receive a padded Buffer, then the padding bytes can be included, but it is not a requirement. The protocol requires that padding bytes to a minimum multiple of 8 be written nonetheless.

Closes #4370 from wesm/ARROW-412 and squashes the following commits:

0da9d8304 <Wes McKinney> Clarify per mailing list discussion
0b590559d <Wes McKinney> Add language about serialized Buffer metadata

Authored-by: Wes McKinney <wesm+git@apache.org>
Signed-off-by: Wes McKinney <wesm+git@apache.org>