Provide information about first message sent after WebSocket subscription

[johnathanDOS]

The first message that Misty sends when you subscribe to a WebSocket from a remote device contains meta information about that WebSocket connection. It does not contain any information related to the sensor/data event that the subscription is for. This causes confusion; developers may assume this first message contains event-specific data and write code that doesn't work.

We should add a note to this page describing the nature of this first message and how to use it or ignore it as needed.

[johnathanDOS]

This message is typically along the lines of:

BatteryCharge {"eventName":"BatteryCharge","message":"Registration Status: API event registered."} (for BatteryCharge data)