Add a javadoc explanation for ProgressiveDownloader that explains that it tries to skip downloading already cached bytes.
download calls through to CacheUtil#cache, which handles skipping already downloaded bytes.
It is useful for callers to know that ProgressiveDownloader has this behavior - otherwise, they might reimplement the logic themselves.
I hope this behavior is something that can be part of the public API :)
PiperOrigin-RevId: 274202995
