Clarify JVMTI heap functions spec about virtual thread stack.

Specification of FollowReferences function says:
 The heap root are the set of system classes, JNI globals, references from thread stacks, and other objects used as roots for the purposes of garbage collection.
But virtual threads and their stacks are deliberately not roots.
Need to update the spec to reflect this.
The same issue with IterateOverReachableObjects spec:
 The root objects comprise the set of system classes, JNI globals, references from thread stacks, and other objects used as roots for the purposes of garbage collection.

Also need to update description of JVMTI_HEAP_REFERENCE_STACK_LOCAL, JVMTI_HEAP_REFERENCE_JNI_LOCAL values as their descriptions says "Heap root reference".