RFR: 8333229: Parallel: Rename ParMarkBitMap::_region_start to _heap_start [v2]

[Albert Mingkun Yang]

> Trivial renaming to better capture that this class corresponds to the whole heap mem-region.

Albert Mingkun Yang has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains two commits:

 - comment
 - pgc-rename-heap

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Changes: https://git.openjdk.org/jdk/pull/19468/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19468&range=01
  Stats: 39 lines in 4 files changed: 0 ins; 13 del; 26 mod
  Patch: https://git.openjdk.org/jdk/pull/19468.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19468/head:pull/19468

PR: https://git.openjdk.org/jdk/pull/19468