RFR: 8305892: G1: Fix G1MMUTracker::when_sec documentation

[Albert Mingkun Yang]

Trivial comment-only change.

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Commit messages:
 - g1-mmu-doc

Changes: https://git.openjdk.org/jdk/pull/13441/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13441&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305892
  Stats: 8 lines in 1 file changed: 0 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/13441.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13441/head:pull/13441

PR: https://git.openjdk.org/jdk/pull/13441