RFR: 8371465: Parallel: Revise asserts around heap expansion

[Albert Mingkun Yang]

Revise heap/old-gen expansion asserts/comments to cover two paths:

1. startup: no gc can occur
2. after-startup: during safepoint, by vm-thread.

Test: tier1-4

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Commit messages:
 - pgc-assert-relax

Changes: https://git.openjdk.org/jdk/pull/28266/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28266&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371465
  Stats: 40 lines in 4 files changed: 19 ins; 18 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/28266.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28266/head:pull/28266

PR: https://git.openjdk.org/jdk/pull/28266