docs: don't show fully hidden backend options in the backend docs

Backend options with Hide: fs.OptionHideBoth are hidden from the config
wizard and from the command line flag listing, but "rclone help backend"
(and therefore the autogenerated backend documentation) still showed
them. Skip them there too so fully hidden options no longer appear in
the docs.

cmd/help.go

@@ -292,6 +292,10 @@ func showBackend(name string) {
 			continue
 		}
 		done[opt.Name] = struct{}{}
+		// Skip options hidden from both the command line and the configurator
+		if opt.Hide&fs.OptionHideBoth == fs.OptionHideBoth {
+			continue
+		}
 		if opt.Advanced {
 			advancedOptions = append(advancedOptions, opt)
 		} else {