The backport workflow should provide instructions to the use when the automated backport fails

- [ ] This issue is blocking
- [ ] This issue is causing unreasonable pain

When the backport action fails (often due to a merge conflict) the user is given a message like the one in https://github.com/dotnet/sdk/pulls/ar-may#issuecomment-5527755454

* failure notification
* raw `git am` command output

For experienced users this is often enough to go on, but users that haven't done many manual backports are confused about where to begin.

The command should add a section that gives step by step instructions to the user about how to manually backport. Those steps should roughly be what the backport action itself would do

* checkout the target branch
* get the patchset for the PR in question
* apply the patchset
* fixup any conflicts
* instructions on how to push the branch up and create the PR (including labels, etc)

This section should be provided after the initial error message and before the `git am` command failure details, because those details are most likely less interesting to the reader.