RFR: JDK-8310676: add note about unnamed module to Elements.getAllModuleElements

[Joe Darcy]

Simple doc clarification, also intended to backport to JDK 21.

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Commit messages:
 - JDK-8310676: add note about unnamed module to Elements.getAllModuleElements

Changes: https://git.openjdk.org/jdk/pull/14633/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14633&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/jdk/pull/14633.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14633/head:pull/14633

PR: https://git.openjdk.org/jdk/pull/14633