RFR: 8368954: G1: Document why G1 uses TLS storage for the current card table reference

[Thomas Schatzl]

Hi all,

  please review some documentation update why G1 now uses TLS storage to get the current card table base value for the card mark.

TLDR: it's the overall fastest currently available way.

Testing: local compilation, this is just a trivial documentation change.

Thanks,
  Thomas

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Commit messages:
 - 8368954

Changes: https://git.openjdk.org/jdk/pull/27573/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27573&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368954
  Stats: 9 lines in 1 file changed: 9 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/27573.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27573/head:pull/27573

PR: https://git.openjdk.org/jdk/pull/27573