RFR: 8303563: GetCurrentThreadCpuTime and GetThreadCpuTime need further clarification for virtual threads

[Serguei Spitsyn]

This is a follow-up to [JDK-8302615](https://bugs.openjdk.org/browse/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.

In the functions `GetCurrentThreadCpuTime` and `GetThreadCpuTime`:

The fragment:
`"The current thread may not be a virtual thread. Otherwise, the error code"`

is replaced with:

"An implementation is not required to support this function
 when the current thread is a virtual thread, in which case"


CSR:  [JDK-8305617](https://bugs.openjdk.org/browse/JDK-8305617)

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Commit messages:
 - 8303563: GetCurrentThreadCpuTime and GetThreadCpuTime need further clarification for virtual threads

Changes: https://git.openjdk.org/jdk/pull/13338/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13338&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303563
  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/13338.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13338/head:pull/13338

PR: https://git.openjdk.org/jdk/pull/13338