doc(storage): improve retry package docs (#5300)
doc(storage): improve retry package docs (#5300)

Add more information to package docs about retry strategy, and
give a short example. This will appear in the docs at
https://pkg.go.dev/cloud.google.com/go/storage
googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go