Issue 11834: std.net.curl abuses ddoc params section and causes ddoc warnings

Fixes: https://issues.dlang.org/show_bug.cgi?id=11834

Basically, the `Params:` section in `onProgress`'s ddoc was abused to document parameters that will be passed to the callback registered by `onProgress`. This confuses ddoc to spew warning messages, and also produces deceptive documentation that looks like those parameters are being passed to `onProgress`, whereas they are actually what *will* be passed *by* `onProgress` to `callback`.