clarify clGetEventProfilingInfo error condition (#783)

Prior to OpenCL 3.0, implementations may return CL_PROFILING_INFO_NOT_AVAILABLE
for an event created by clEnqueueSVMFree.

Author: bashbaug

api/opencl_runtime_layer.asciidoc

@@ -9359,6 +9359,9 @@ Otherwise, it returns one of the following errors:
     not set for the command-queue, if the execution status of the command
     identified by _event_ is not {CL_COMPLETE} or if _event_ is a user event
     object.
+    Prior to OpenCL 3.0, implementations may return
+    {CL_PROFILING_INFO_NOT_AVAILABLE} for an event created by
+    {clEnqueueSvmFree}.
   * {CL_INVALID_VALUE} if _param_name_ is not valid, or if size in bytes
     specified by _param_value_size_ is < size of return type as described in
     the <<event-profiling-info-table,Event Profiling Queries>> table and