[jdk21] RFR: 8310676: add note about unnamed module to Elements.getAllModuleElements

[Joe Darcy]

Doc-only backport.

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Commit messages:
 - Backport 69f3114c4119d66a8c16cf95c35ea1adc488bdee

Changes: https://git.openjdk.org/jdk21/pull/61/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21&pr=61&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310676
  Stats: 5 lines in 1 file changed: 5 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk21/pull/61.diff
  Fetch: git fetch https://git.openjdk.org/jdk21.git pull/61/head:pull/61

PR: https://git.openjdk.org/jdk21/pull/61