Clarifying behavior of /renter/upload API
Created by: mtlynch
The API documentation does not make it clear that it returns when the upload starts rather than finishes. This recently created confusion with the minio integration:
https://github.com/minio/minio/pull/5233#commitcomment-25836470
Fixes some links in Renter.md. Removes a redundant check from validateSiapath().