Doc and comment patch for packed varlena

Patch applied. Thanks.

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Gregory Stark wrote:

Caught someone using PG_DETOAST_PACKED and VARDATA_ANY on a structure that
required alignment so I guess some more prominent warnings are in order. I
also added a paragraph to the "User-Defined Types" chapter on using these
macros since it seems like they're a hit.

[ Attachment, skipping... ]

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Gregory Stark
EnterpriseDB http://www.enterprisedb.com

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