docs: add a note about the B2 download_url format

Currently the B2 docs don't specify which format the download_url
setting should have, and if you input it wrong, there is nothing
in the verbose logs or anywhere else that can let you know that.
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f