Add example of decls you could move

[keith]

this is based on this conversation https://github.com/lyft/swift-index-store/pull/43#issuecomment-1569082087

this doesn't really work well right now because it would require more filtering to understand what types of declarations could actually be moved. Like a property of a struct couldn't be moved even if the only users were in another module unless the whole struct could be

[maxwellE]

This helps, I am still trying to learn how to structure my thinking when using indexstores. Thank you