LAVA-1107 fix "device-types add" documentation
LAVA-1107 fix "device-types add" documentation

We can either add one specific device-type or all.

Change-Id: Idebc7b665211a65be49f53e1e84cd7c78967d045

Linaro/lava-server,Linaro/lava-server,Linaro/lava-server,Linaro/lava-server