Document types

Values of doc_type in Create a document. Documents are returned with the same value.

`doc_type`	Document	Invoicing-service name
`100`	Work order	`order`
`200`	Delivery note	`delcert`
`300`	Transaction invoice	`deal`
`305`	Tax invoice	`invoice`
`320`	Invoice receipt	`invrec`
`330`	Credit invoice (refund)	`refund`
`400`	Receipt	`receipt`
`405`	Donation receipt	`trec`
`600`	Price offer	`offer`