RFR: Note on Affected version

[Jesper Wilhelmsson]

Added a note on when to use the affected version field and did some other minor cleanups.

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Commit messages:
 - Changed wording
 - Note on Affected version

Changes: https://git.openjdk.org/guide/pull/140/files
  Webrev: https://webrevs.openjdk.org/?repo=guide&pr=140&range=00
  Stats: 10 lines in 2 files changed: 5 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/guide/pull/140.diff
  Fetch: git fetch https://git.openjdk.org/guide.git pull/140/head:pull/140

PR: https://git.openjdk.org/guide/pull/140