Clarify clGetDeviceIDs(CL_DEVICE_TYPE_DEFAULT) behavior when there is more than one custom device

Hi,

The spec seems to be contradictory when there are several devices in the platform and all of them are custom:

> One device in the platform must be returned as the `CL_DEVICE_TYPE_DEFAULT` device when passed as the *device_type* to **clGetDeviceIDs**.

> The default OpenCL device must not be a `CL_DEVICE_TYPE_CUSTOM` device unless it is the only device in the platform.

If I read it correctly, an implementation is required to return both one and no devices.