Unified API spec: Consider thread-safety description

The description of which API entry points are thread-safe (or not) was made much clearer in the 1.1 specification, but the unified API spec should still describe the 1.0 case.

A concrete case where this matters is the deprecated clSetCommandQueueProperty function. During review of the initial unified API PR @bashbaug suggested [1] taking the wording already present in the OpenCL unified headers:

    /*
     *  WARNING:
     *     This API introduces mutable state into the OpenCL implementation. It has been REMOVED
     *  to better facilitate thread safety.  The 1.0 API is not thread safe. It is not tested by the
     *  OpenCL 1.1 conformance test, and consequently may not work or may not work dependably.
     *  It is likely to be non-performant. Use of this API is not advised. Use at your own risk.
     *
     *  Software developers previously relying on this API are instructed to set the command queue
     *  properties when creating the queue, instead.
     */

[1] https://github.com/KhronosGroup/OpenCL-Docs/pull/78#discussion_r334115134