RFR: Clarifications on backport process

[Jesper Wilhelmsson]

More clarifications needed on the new "no forwardport" policy.

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Commit messages:
 - Clarifications on backport process

Changes: https://git.openjdk.org/guide/pull/108/files
 Webrev: https://webrevs.openjdk.org/?repo=guide&pr=108&range=00
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/guide/pull/108.diff
  Fetch: git fetch https://git.openjdk.org/guide.git pull/108/head:pull/108

PR: https://git.openjdk.org/guide/pull/108