Make the HTTP mode + poll more clear

docs/http-api-mode.md

@@ -28,7 +28,7 @@ services:
       - 8080:8080
 ```
 
-By default, enabling this mode prevents periodic polls (i.e. what is specified using `--interval` or `--schedule`). To run periodic updates regardless, pass `--http-api-periodic-polls`.
+By default, enabling this mode prevents periodic polls (i.e. what is specified using `--interval` or `--schedule`). To run periodic updates regardless, _additionally_ pass in the switch `--http-api-periodic-polls`.
 
 Notice that there is an environment variable named WATCHTOWER_HTTP_API_TOKEN. To prevent external services from accidentally triggering image updates, all of the requests have to contain a "Token" field, valued as the token defined in WATCHTOWER_HTTP_API_TOKEN, in their headers. In this case, there is a port bind to the host machine, allowing to request localhost:8080 to reach Watchtower. The following `curl` command would trigger an image update: