RFR: 8272235: G1: update outdated code root fixup

[Albert Mingkun Yang]

Simple change of updating outdated comments and certain renaming.

Test: hotspot_gc

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Commit messages:
 - nmethod-list

Changes: https://git.openjdk.java.net/jdk/pull/5073/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5073&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272235
  Stats: 19 lines in 4 files changed: 7 ins; 0 del; 12 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5073.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5073/head:pull/5073

PR: https://git.openjdk.java.net/jdk/pull/5073