RFR: Clarified backport docs [v2]

[Jesper Wilhelmsson]

> Explaining the difference between jdk21u and jdk21u-dev.

Jesper Wilhelmsson has updated the pull request incrementally with one additional commit since the last revision:

  Fixed bug

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Changes:
  - all: https://git.openjdk.org/guide/pull/158/files
  - new: https://git.openjdk.org/guide/pull/158/files/aaf205ef..bc67d331

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=guide&pr=158&range=01
 - incr: https://webrevs.openjdk.org/?repo=guide&pr=158&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 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