RFR: 8371788: Fix documentation for CollectedHeap::collect(GCCause)

[Thomas Schatzl]

Hi all,

  please review this trivial fix of the documentation of `Collectedheap::collect()` and all its overrides to indicate that it's not just used for `System.gc()` and will do a full collection.

Testing: local compilation

Thanks,
  Thomas

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Commit messages:
 - 8371788

Changes: https://git.openjdk.org/jdk/pull/28289/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28289&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371788
  Stats: 10 lines in 4 files changed: 0 ins; 9 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/28289.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28289/head:pull/28289

PR: https://git.openjdk.org/jdk/pull/28289