RFR: 8366157: Clarify in man pages that only G1 supports MaxGCPauseMillis

[Jonas Norlinder]

Hi all,

This patch clarifies the description of `MaxGCPauseMillis`.

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Commit messages:
 - Clarify MaxGCPauseMillis

Changes: https://git.openjdk.org/jdk/pull/26940/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26940&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8366157
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/26940.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26940/head:pull/26940

PR: https://git.openjdk.org/jdk/pull/26940