RFR: Clarified the Affects version section

[Joakim Nordström]

Clarification for the [Affects version section](https://openjdk.org/guide/#usage-of-the-rel-na-label)

* Removed some newlines so things are kept in one paragraph
* Changed some wording to make the _don'ts_ a bit clearer

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Commit messages:
 - Clarified the Affects version section

Changes: https://git.openjdk.org/guide/pull/170/files
  Webrev: https://webrevs.openjdk.org/?repo=guide&pr=170&range=00
  Stats: 15 lines in 1 file changed: 0 ins; 10 del; 5 mod
  Patch: https://git.openjdk.org/guide/pull/170.diff
  Fetch: git fetch https://git.openjdk.org/guide.git pull/170/head:pull/170

PR: https://git.openjdk.org/guide/pull/170