[code-reflection] RFR: Make CodeContext a final class

[Paul Sandoz]

Make `CodeContext` a final class, since it only has one implementation.

Update doc/terminology.

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Commit messages:
 - Make CodeContext a final class.

Changes: https://git.openjdk.org/babylon/pull/887/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=887&range=00
  Stats: 457 lines in 6 files changed: 165 ins; 221 del; 71 mod
  Patch: https://git.openjdk.org/babylon/pull/887.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/887/head:pull/887

PR: https://git.openjdk.org/babylon/pull/887