Rewrite migration notes for [ticket:3514]

The change to "evaluates none" datatypes in the ORM was
not fully described in the migration notes, missing the
key behavioral change that a column which is missing a default
entirely will not receive a value for a missing JSON column now.
The issue here touched upon a revisit of the assumptions
in [ticket:3514], but overall the old behavior "worked" mostly
because the ORM wants to explicitly render NULL into an INSERT
for column values that are missing, which itself is a legacy
behavior which should be considered for possible removal in
a future major release.  Given that "missing ORM value + no
column default set up == dont put it in the INSERT" would be
the most intuitive behavior, the move in [ticket:3514] represents
a step in this direction.

Change-Id: I454d5bb0773bd73d9864925dcc47f1f0810e33ba
Fixes: #3830

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