Clarify local storage vs temp storage in WH uploads

What does this MR do and why?

A small follow-up to #351657 (closed)

I wanted to clarify in code the distinction between Workhorse using local storage for uploads in these two cases:

• It temporarily buffers a file to local or shared storage, even if object storage is available. This can happen e.g. because direct_upload is disabled, or unsupported for this particular upload.
• It uploads to local storage as the final destination. This is in fact unsupported as of now, but we are considering to add this. In this case object storage is not enabled or available but WH might still upload to it directly.

This had confused me when looking at this code because IsLocal suggested to me that WH was already doing the latter when in fact it didn't.

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