Clarify in documentation and console message that DepotDownloader will not download files if a manifest is present, or allow users to re-download files.

[ngtban]

If I read ContentDownloader.cs correctly then DD will not download files if a manifest is present. Unfortunately this is not clear from the console message, and this behaviour is not documened in the README. I think at least this behaviour should be documented.

[ryantheleach]

@ngtban I think this is possibly related to https://github.com/SteamRE/DepotDownloader/issues/317