RFR: 8295838: Document why we do not print Code Cache Roots phase in G1 logging

[Thomas Schatzl]

Hi all,

  can I get reviews for this trivial addition of some comment why we do not measure the `Code Cache Roots` phase in G1 logging?

Testing: local compilation

Thanks,
  Thomas

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Commit messages:
 - add comment about why we do not measure code cache roots

Changes: https://git.openjdk.org/jdk/pull/10835/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10835&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295838
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10835.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10835/head:pull/10835

PR: https://git.openjdk.org/jdk/pull/10835