RFR: Clarified that for enhancements the affects version should normally be empty

[calnan]

Unlike for bugs, for enhancements the affectsversion is less relevant as vast majority will be fixed in a future release and will not be backported. There are situations where it is relevant, if the code is only in an LTS release for example.

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Commit messages:
 - Clarified for enhancements the affects version should normally be empty

Changes: https://git.openjdk.org/guide/pull/115/files
 Webrev: https://webrevs.openjdk.org/?repo=guide&pr=115&range=00
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/guide/pull/115.diff
  Fetch: git fetch https://git.openjdk.org/guide.git pull/115/head:pull/115

PR: https://git.openjdk.org/guide/pull/115