docs: fix backend doc generator to not output duplicate config names

This was always the intention, it was just implemented wrong.

This shortens the s3 docs by 1369 bringing them down to half the size
just about.

Fixes #7325

cmd/help.go

@@ -309,6 +309,7 @@ func showBackend(name string) {
 		if _, doneAlready := done[opt.Name]; doneAlready {
 			continue
 		}
+		done[opt.Name] = struct{}{}
 		if opt.Advanced {
 			advancedOptions = append(advancedOptions, opt)
 		} else {