Today if a user issues the "/backport" command then the bots will suggest a PR body with information about the original commit. However a user might choose to write something else for the PR body and then reviewers of the backport PR will _not_ see any information about the original commit. It would therefore be helpful if the bots added a comment to a backport PR with information about the original commit.

The drawback will be if a user does use the PR body suggested by the bots, then information about the original commit will be present in both the PR body and in the comment added by the bots. Personally I think this is worthwhile tradeoff, it is better to have this information twice in some PRs than to not have it all in some PRs.

Suggested markdown for the comment (this is the exact same wording that the bots suggest as PR body):

This pull request contains a backport of commit [f0a6e71e](https://git.openjdk.org/jdk/commit/f0a6e71e4d63c9820659f6ff29f94d0476d48b09) from the [openjdk/jdk](https://git.openjdk.org/jdk) repository. The commit being backported was authored by Sergey Bylokhov on 10 Nov 2022 and was reviewed by Erik Joelsson, Phil Race and Magnus Ihse Bursie.