The existing core Link module does not actually support attributes -- the widget does not display editing forms for attributes, and if a contrib widget did provide attributes, it looks like the 'target' attribute set on the individual field data item would be overwritten by link_field_formatter_prepare_view() [that is probably a bug that should be remedied?].
So I think that the module's .info description and help should not say it supports attributes -- this is misleading. It should either take that out of the description or it should say you would need a hypothetical (currently nonexistent) contrib module to provide a widget that would actually support editing attributes.
Better yet, it should actually support some attributes, such as rel and target. The field is IMO kind of useless for practical applications without this.