GetCurrentThreadCpuTime and GetThreadCpuTime need further clarification for virtual threads

This is a follow-up to JDK-8302615 where GetCurrentThreadCpuTime and GetThreadCpuTime were changed from being not supported to optional, when called from/with a virtual thread. There are two additional sentences that need adjustment to avoid creating a conflict in the spec.

--- a/src/hotspot/share/prims/jvmti.xml
+++ b/src/hotspot/share/prims/jvmti.xml
@@ -11022,7 +11022,8 @@ myInit() {
             the current thread (see
             <functionlink id="GetCurrentThreadCpuTimerInfo"/> vs
             <functionlink id="GetThreadCpuTimerInfo"/>).
- The current thread may not be a virtual thread. Otherwise, the error code
+ An implementation is not required to support this function
+ when the current thread is a virtual thread, in which case
             <errorlink id="JVMTI_ERROR_UNSUPPORTED_OPERATION"></errorlink> will be returned.
             On many platforms this call will be equivalent to:
 <example>
@@ -11125,7 +11126,8 @@ myInit() {
           <jthread null="current" impl="noconvert"/>
             <description>
               The thread to query.
- The <code>thread</code> may not be a virtual thread. Otherwise, the error code
+ An implementation is not required to support this function
+ when the given thread is a virtual thread, in which case
                <errorlink id="JVMTI_ERROR_UNSUPPORTED_OPERATION"></errorlink> will be returned.
              </description>
         </param>