Better documentation for insert-replace

[simonw]

Currently: https://sqlite-utils.datasette.io/en/stable/python-api.html#insert-replacing-data

If you want to insert a record or replace an existing record with the same primary key, using the replace=True argument to .insert() or .insert_all():

Should describe the exception you get first, then how to use replace to avoid it.

[simonw]

Much better: https://sqlite-utils.datasette.io/en/latest/python-api.html#insert-replacing-data