schedule-revdeps documentation could be clearer

I think this line in the docs is not clear.

9. Create DB entries for all the things we want to rebuild: `python -m rebuilds schedule-revdeps foo-dev --local-versions`

I was able to interpret that I needed to substitue the -dev package for the package I wanted for foo-dev. poppler is an interesting case because it has multiple -dev packages. So I ran that command one time for each -dev package. This appeared to work correctly.