RFR: 8306027: Clarify JVMTI heap functions spec about virtual thread stack. [v2]

[Alex Menkov]

> The fix updates JVMTI spec updates description of heap functions to support virtual threads.
> Virtual threads are not heap roots by design, so FollowReference/IterateOverReachableObjects specs are updated to note only platform threads.
> References from thread stacks (including virtual threads) are reported as JVMTI_HEAP_REFERENCE_STACK_LOCAL/JVMTI_HEAP_REFERENCE_JNI_LOCAL, so description of the values is relaxed.

Alex Menkov has updated the pull request incrementally with one additional commit since the last revision:

  updated spec to follow CSR

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/13661/files
  - new: https://git.openjdk.org/jdk/pull/13661/files/8d9e284e..6fd16ef9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13661&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13661&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13661.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13661/head:pull/13661

PR: https://git.openjdk.org/jdk/pull/13661