To delete the related records manually, without changing the foreign key to use the CASCADE option, you can do it in two steps:
delete from doc_source where document_id = 209965;
delete from documents where document_id = 209965;
You may want to wrap that in a transaction and confirm that you've only deleted the records from
doc_source
that you intended to, before you delete from
documents
. If this is
Important Data, then perhaps get a backup of the data before you proceed. If this is only a database you are using for educational purposes, and can repopulate easily, you might be able to afford to skip the backup.