RFR: 8327788: G1: Improve concurrent reference processing documentation

Mon Mar 11 11:05:01 UTC 2024

Simple doc update re discovered ref in concurrent ref-processing.

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Commit messages:
 - g1-cm-ref

Changes: https://git.openjdk.org/jdk/pull/18188/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18188&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327788
  Stats: 6 lines in 1 file changed: 2 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/18188.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18188/head:pull/18188

PR: https://git.openjdk.org/jdk/pull/18188