Further clarify instructions for options after "Compare" button in GUI section

If it is possible (even if unlikely) for someone's branch to be behind, hit the 'Compare' button, and not get the **there isn’t anything to compare** message, would it helpful to include a line about what to do next in that case?

From @dtwrub : I want to say in this specific case, it's unlikely - this is in case you have already forked a repo and haven't done anything with it in a while. I guess it might be possible, if you made some changes to your repo, but didn't push them to the origin?

From @fmichonneau: If I follow correctly, I think here you'd want to add how to bring changes from upstream to your repository using the web interface. I have never done this, but I think you'd need to create a pull request using the upstream repository as the head fork and your repository as the base.