[jdk21u] RFR: 8316906: Clarify TLABWasteTargetPercent flag

[Ludvig Janiuk]

Backporting a comment for a GC flag

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Commit messages:
 - Backort 77fac0f4c6692bcc06456d03a4329d9f8ddeeded

Changes: https://git.openjdk.org/jdk21u/pull/225/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=225&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316906
  Stats: 7 lines in 1 file changed: 5 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u/pull/225.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/225/head:pull/225

PR: https://git.openjdk.org/jdk21u/pull/225