RFR: 2138: Backport instructions should include suggested PR comment

[Zhao Song]

In this patch, if user use the backport command to create a backport, regardless the backport can be created automatically or not, the user will get a suggestion for the pull request body.

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Commit messages:
 - SKARA-2138

Changes: https://git.openjdk.org/skara/pull/1597/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1597&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2138
  Stats: 72 lines in 3 files changed: 38 ins; 33 del; 1 mod
  Patch: https://git.openjdk.org/skara/pull/1597.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1597/head:pull/1597

PR: https://git.openjdk.org/skara/pull/1597