Clarify presence of old values when not using REPLICA IDENTITY FULL

In the PG connector docs we currently say:

DEFAULT - UPDATE and DELETE events will only contain the previous values for the primary key columns of a table

But actually, the PK values will not be provided when just another column is updated. This is caused by the behavior documented upstream:

In all cases, no old values are logged unless at least one of the columns that would be logged differs between the old and new versions of the row.