strengthen requirements for CL_DEVICE_TYPE query

[bashbaug]

Proposed fix for #224.

This PR:

1. Moves the requirement that CL_DEVICE_TYPE_CUSTOM must not be combined with any other device types to the description of CL_DEVICE_TYPE. (see (4) below)
2. Adds the requirement that CL_DEVICE_TYPE_DEFAULT must only be used to get device IDs using clGetDeviceIDs and must NOT be returned as the CL_DEVICE_TYPE for any device.
3. Clarifies that CL_DEVICE_TYPE_ALL also must only be used to get device IDs using clGetDeviceIDs.
4. Adds an informative note that most implementations should only return a single device type for CL_DEVICE_TYPE, and should not return a combination of device types. Updated: Adds the requirement that the device must return only a single device type for CL_DEVICE_TYPE.

[alycm]

Textually LGTM. Group needs to approve overall change.

[alycm]

New wording still approved, it's very clear on returning a single value.

[bashbaug]

Had a thought: Would we want CL_DEVICE_TYPE_DEFAULT and CL_DEVICE_TYPE_ALL to be valid values to pass to clCreateContextFromType? I think we would.

If so, I should adjust the descriptions of these devices types so it is clearer that this is allowed. In other words, CL_DEVICE_TYPE_ALL could be passed to clGetDeviceIDs or to clCreateContextFromType, though it will never be returned as a device type for CL_DEVICE_TYPE.

[bashbaug]

I've updated this PR so it describes clCreateContextFromType also. I think it's ready to go.