RFR: clarify how to set the affects versions

[calnan]

moved together information on how the set the affects version and the meaning of the (Rel)-na and (Rel)-wnf labelss

-------------

Commit messages:
 - clarify how to set the affects versions

Changes: https://git.openjdk.org/guide/pull/119/files
 Webrev: https://webrevs.openjdk.org/?repo=guide&pr=119&range=00
  Stats: 53 lines in 2 files changed: 44 ins; 1 del; 8 mod
  Patch: https://git.openjdk.org/guide/pull/119.diff
  Fetch: git fetch https://git.openjdk.org/guide.git pull/119/head:pull/119

PR: https://git.openjdk.org/guide/pull/119