Clarify the reason temporary unique collection names are made

We use "makeUniqueCollectionNamespace" to make collection namespaces with random unused names. We often call them "tmp<random>", which doesn't say what they're used for (https://github.com/mongodb/mongo/blob/cdef5d9ad7a5c138637354a0644457a439507b80/src/mongo/db/catalog/create_collection.cpp#L165). It would be helpful if we added a prefix or suffix to each of these to say if it's for a create, rename, rollback, or convertToCapped.