RFR: 8337887: [JVMCI] Clarify jdk.vm.ci.code.Architecture.getName javadoc

[Doug Simon]

This PR improves the documentation of `jdk.vm.ci.code.Architecture.getName`.

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Commit messages:
 - improve documentation of getName for each JVMCI Architecture class

Changes: https://git.openjdk.org/jdk/pull/20476/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20476&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337887
  Stats: 15 lines in 4 files changed: 11 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/20476.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20476/head:pull/20476

PR: https://git.openjdk.org/jdk/pull/20476