- Document the most transparent way to set JSON.NULL for

- Document the most transparent way to set JSON.NULL for
a column default.

The JSON.NULL value is special in that while it is a Python-side
value, it represents "do this special behavior", and is not at all
like any other Python-side value for which normally, that's exactly
the type of data we want back.   So in this case, encourage the user
to use a SQL expression that is exact, so in the ORM context, you get
back what was actually persisted in the column.  There's some variants
of this such as literal(JSON.NULL, JSON) but text("'null'") is simpler.

Change-Id: I0339fafdc03e4b2f533d53970b2f74c774f2687b
Fixes: #3923
(cherry picked from commit 4d4ea792e7ea7945de9c9b1e005ce09b2feb45b5)