[jdk21] RFR: 8254566: Clarify the spec of ClassLoader::getClassLoadingLock for non-parallel capable loader

[Mandy Chung]

Backport of commit [b9198f99](https://github.com/openjdk/jdk/commit/b9198f993173b0fbdc7ff1400e2205d3969370da).

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Commit messages:
 - Backport b9198f993173b0fbdc7ff1400e2205d3969370da

Changes: https://git.openjdk.org/jdk21/pull/93/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21&pr=93&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8254566
  Stats: 16 lines in 1 file changed: 9 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/jdk21/pull/93.diff
  Fetch: git fetch https://git.openjdk.org/jdk21.git pull/93/head:pull/93

PR: https://git.openjdk.org/jdk21/pull/93