RFR: 8277336: Improve CollectedHeap::safepoint_workers comments

[Albert Mingkun Yang]

Simple change of updating comments.

Test: build

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Commit messages:
 - safepoint_workers_comment

Changes: https://git.openjdk.java.net/jdk/pull/6432/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6432&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277336
  Stats: 9 lines in 1 file changed: 1 ins; 0 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6432.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6432/head:pull/6432

PR: https://git.openjdk.java.net/jdk/pull/6432