Document adding a timestamp column with SQL `NOW()` as default value

Is it possible to add a timestamp typed column to an existing table, with the default value being a raw SQL `NOW()`? So if a new record is added, the column value is set to the timestamp the column was added.

Start by locating Avram's migration API and the PostgreSQL column-default handling. Check how existing-table timestamp columns and raw SQL defaults are represented, then run the relevant migration tests or add coverage showing that new records receive the database timestamp after the column is added.

Written by the indexing model from the issue text.