Add some useful notes in  devref/db_layer.rst
Add some useful notes in  devref/db_layer.rst

Added information about difference between default and server_default
parameters for columns.

This was suggested in comments to I466202ac8c378f1e19e42ec2880d0c8fbc35fd28.

Related-bug: #1295539

Change-Id: I605ab8a4f3af80497fb8ad21d43fa13d8d533325

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