You can crawl only supported databases that you can connect to by using JDBC driver by using the Admin console. You can also write a custom applications to add the "document" to the index using the "Add and delete document API". The details of this can be found in the OYE API guide
http://842docs.omnifind.info/oye_api_guide_v842.pdfWith regards to your other question of "what is considered as a "document" when crawling and indexing data from a database" I honestly don't know. Maybe Sean or Maxime could throw some light here.