docs: Make the leading slash requirement for filepath clear

Filepath currently requires a leading slash, most likely because the trailing slash after downloads is missing somewhere.

While we wait for this bug to be fixed (if ever, due to backwards compatibility), make this extremely clear in the documentation.

Technically, this is not just a documentation clarification, but also a documentation bug, as the link is not ...downloads/:filepath which would indicate the result would resolve to ...downloads//<path>.

Fixes: #352947