Clarify INSERT/UPDATE defaults vs. data marshalling

- Add a full introductory paragraph to INSERT/UPDATE defaults
stating what we mean when we talk about this concept.  Add a note
differentiating what a default does, vs. a rule that intercepts
data as it moves into the database, providing links

- Add a quick section referring to TypeDecorator in the ORM
section on modifying attribute behavior

- Add an "ORM tip", a new thing that we can use in Core to link
to ORM concepts when useful, in the TypeDecorator section which
mentions that for more open ended conversion of arbitrary user
data based on business rules, @validates might be useful
(although this still does not suit the case of full blown form
validation).

- add glossary entries for DML, data marshalling since we already
use these terms and I'd like to refer to them more often.

Fixes: #4796
Change-Id: Ic4cd5c1595fd51c00a9bf24c021d2e56d457d346

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