Update upsert docs to make it clear of the new behavior
Update upsert docs to make it clear of the new behavior

See: https://github.com/yesodweb/persistent/issues/613

paul-rouse/persistent,naushadh/persistent,erikd/persistent,gbwey/persistent,plow-technologies/persistent,bitemyapp/persistent,psibi/persistent,yesodweb/persistent,creichert/persistent,gbwey/persistent,gbwey/persistent