Document Requirement for CL_FILTER_LINEAR and GPUs

For conformance devices that report as GPU are not required to pass the `CL_FILTER_LINEAR` tests, see https://github.com/KhronosGroup/OpenCL-CTS/issues/829. The OpenCL C specification should state what this means.

E.g. should it be possible to use `CL_FILTER_LINEAR` on any device type, but the precision on a GPU device is unspecified.

Start by reading the OpenCL C specification requirements for CL_FILTER_LINEAR and the linked OpenCL-CTS issue 829. Determine the intended requirement for conformance devices reporting as GPUs and clarify whether use is permitted with unspecified precision. Done means the specification explicitly states the device-type requirement and precision behavior.

Written by the indexing model from the issue text.