Document SQLite "mixed binary" behavior

Document SQLite "mixed binary" behavior

The Pysqlite driver can store a string value with
or without an indicator that the value is to be retrieved
as bytes or as a unicode string object.    To suit the
use case where a SQLite database has mixed values on a row
by row basis, provide a recipe for a MixedBinary datatype.

Change-Id: I9a166bd6fc673d8d46a53ab9697cb3d412e5fcee
References: #5073
(cherry picked from commit ee1d914888113ceb9928ece6e0a715c813bdfcfa)