Can I delete file (document) described as an iPhoto library that was already migrated to photos?

Yes, it is not necessary to keep the iPhoto Library on your system drive, if you migrated it to

photos and are happy with the result. But I would keep a copy of the old iPhoto Library on an external volume, as a safety copy of the original image files and the videos. I have frequently come across older images and videos in Photos, that have not been migrated correctly and needed recovering from the old IPhoto Library. It is hard to discover this instantly after the migration, because we will 9nly notice missing or damaged files, when we are trying to work with a photo or video. The thumbnail of a photo cannot tell us, if the original behind it is all right.