Document notification responses for notifications functions

Functions that send HTTP requests should return the response code, body, headers or all of these. We need to document these responses. These functions include:

- `influxdb.api`
- `http.get`
- `http.post`
- `slack.message`
- `pushbullet.pushData`
- `pushbullet.pushNote`
- `pagerduty.sendEvent`
- `alerta.alert`
- `victorops.event`
- `zenoss.event`
- `discord.send`
- `bigpanda.sendAlert`
- `opsgenie.sendAlert`
- `sensu.event`
- `teams.message`
- `telegram.message`
- `webexteams.message`