RFR: More details for Skara backports

[Jesper Wilhelmsson]

Added some more details around using the /backport command.

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Commit messages:
 - More details for Skara backports

Changes: https://git.openjdk.org/guide/pull/124/files
  Webrev: https://webrevs.openjdk.org/?repo=guide&pr=124&range=00
  Stats: 10 lines in 1 file changed: 9 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/guide/pull/124.diff
  Fetch: git fetch https://git.openjdk.org/guide.git pull/124/head:pull/124

PR: https://git.openjdk.org/guide/pull/124