Flexible-PCBs: what data is needed for manufacturers?
Mostly the same stuff as for rigid FR4 boards. Material will often be polyimide or polyester. Of course you'll want to bend the PCB, so check with the manufacturer what the minimum bend radius is for different thicknesses. In general this radius is small, like 6 to 10 times the circuit's thickness for a double layer FPC, that's a couple mm, and shouldn't be a real limitation for most applications.  

Provide drawings of stiffeners, and their material (often FR4). Also of contact areas, with data on finishing (like gold, and its thickness).