RFR: Clarified backport docs [v3]

[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:

  Updates

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

Changes:
  - all: https://git.openjdk.org/guide/pull/158/files
  - new: https://git.openjdk.org/guide/pull/158/files/bc67d331..59f261bb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=guide&pr=158&range=02
 - incr: https://webrevs.openjdk.org/?repo=guide&pr=158&range=01-02

  Stats: 6 lines in 1 file changed: 0 ins; 1 del; 5 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