Clarify what devices are selected by `clCreateContextFromType`

Currently specified as follows

>To create an OpenCL context from a specific device type, call clCreateContextFromType
>Only devices that are returned by clGetDeviceIDs for device_type are used to create the context. The context does not reference any sub-devices that may have been created from these devices.

A few things to consider (from discussion in 2025/06/17 teleconference)
- What is the behaviour when passing `CL_DEVICE_TYPE_ALL`
- Should the description refer to "all devices that are returned" instead of "only devices that are returned"
- Some reported that the conformance tests did not match the specification. We should make sure they are aligned.
- Anything else?

Start by reviewing the current clCreateContextFromType description and the related clGetDeviceIDs behavior. Check how CL_DEVICE_TYPE_ALL and sub-devices are described, then compare the specification with the conformance tests. Done means the device-selection wording and tests agree.

Written by the indexing model from the issue text.