[jdk17] RFR: JDK-8225667: Clarify the behavior of System::gc w.r.t. reference processing

[Mandy Chung]

This spec clarification is a follow-up to [JDK-8224760](https://bugs.openjdk.java.net/browse/JDK-8224760?focusedCommentId=14268320&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-14268320) w.r.t. reference processing.  Since there is no guarantee for any memory reclamation by the invocation of `System::gc`, the spec should also clarify that there is no guarantee in determining the change of reachability of any objects or any particular number of `Reference` objects be enqueued and cleared.

CSR:
   https://bugs.openjdk.java.net/browse/JDK-8269690

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Commit messages:
 - JDK-8225667: Clarify the behavior of System::gc w.r.t. reference processing

Changes: https://git.openjdk.java.net/jdk17/pull/183/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17&pr=183&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8225667
  Stats: 9 lines in 2 files changed: 9 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk17/pull/183.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17 pull/183/head:pull/183

PR: https://git.openjdk.java.net/jdk17/pull/183