RFR: Clarified backport docs

[Jesper Wilhelmsson]

Explaining the difference between jdk21u and jdk21u-dev.

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Commit messages:
 - Split it up
 - Clarified backport docs

Changes: https://git.openjdk.org/guide/pull/158/files
  Webrev: https://webrevs.openjdk.org/?repo=guide&pr=158&range=00
  Stats: 8 lines in 1 file changed: 4 ins; 2 del; 2 mod
  Patch: https://git.openjdk.org/guide/pull/158.diff
  Fetch: git fetch https://git.openjdk.org/guide.git pull/158/head:pull/158

PR: https://git.openjdk.org/guide/pull/158