Added 'or created using structure()' to warning about invalid self ref.
Added 'or created using structure()' to warning about invalid self ref.

jangorecki/data.table,Rdatatable/data.table,jangorecki/data.table,Rdatatable/data.table,Rdatatable/data.table,jangorecki/data.table,vlulla/data.table,Rdatatable/data.table,vlulla/data.table,vlulla/data.table,jangorecki/data.table,vlulla/data.table