RFR: 8329174: update CodeBuffer layout in comment after constants section moved

[lusou-zhangquan]

Enhancement [JDK-6961697](https://bugs.openjdk.org/browse/JDK-6961697) moved nmethod constants section before instruction section, but the layout scheme in codeBuffer.cpp was not changed correspondingly. The mismatch between layout scheme in source code and actual layout is misleading, so we'd better fix it.

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Commit messages:
 - 8329174: update CodeBuffer layout in comment after constants section moved

Changes: https://git.openjdk.org/jdk/pull/18529/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18529&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329174
  Stats: 9 lines in 1 file changed: 4 ins; 4 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/18529.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18529/head:pull/18529

PR: https://git.openjdk.org/jdk/pull/18529