Documentation misses "Subscriptions send data to <subscription_endpoint_host>/write'

My colleague and me spent the whole day trying to get the InfluxDB to send data to our HTTP-Endpoint. We didn't know that a subscription will get data from our InfluxDB and not send it to the we defined in our CREATE SUBSCRIPTION command, but /write. Would be great to add that info to the docs to save time of future users.

##### Relevant URLs
- https://docs.influxdata.com/influxdb/v1.8/administration/subscription-management/
- _Provide other relevant URLs (project issues, community threads, etc.)_

Start with the InfluxDB v1.8 subscription-management documentation at the relevant URL. Clarify that subscriptions send data to /write rather than only the host specified in CREATE SUBSCRIPTION. The work is done when this endpoint behavior is documented clearly for future users.

Written by the indexing model from the issue text.