Update MQTT JSON downlink according to new requirements

docs/software/mqtt/home-assistant.mdx

@@ -268,7 +268,7 @@ First, create an input text helper entity. The preferred way to configure an inp
 
 ### Create a Send Message Automation
 
-This automation will check the send box for changes.  After typing a message, either hit enter or click off the box and the automation will send a text string in JSON to the mqtt broker. Make sure to update the channel name and device ID in the example below. 
+This automation will check the send box for changes.  After typing a message, either hit enter or click off the box and the automation will send a text string in JSON to the mqtt broker. Make sure to call your channel "mqtt" and to update the device ID and `from` field in the example below. 
 
 ```yaml
 - alias: Meshtastic - Send Automation
@@ -288,9 +288,9 @@ This automation will check the send box for changes.  After typing a message, ei
       data:
         qos: 0
         retain: false
-        topic: msh/2/json/LongFast/!67ea9400
+        topic: msh/2/json/mqtt/!67ea9400
         payload: >-
-          {"channel":"0","sender":"broker","type":"sendtext","payload":"{{    
+          {"from":1743426560,"type":"sendtext","payload":"{{    
           states('input_text.meshtastic_send_box') }}"}
     - delay:
         hours: 0

docs/software/mqtt/index.mdx

@@ -96,11 +96,14 @@ The `from` field can thus be used as a stable identifier for a specific node.  N
 
 If the message received contains valid JSON in the payload, the JSON is deserialized and added as a JSON object rather than a string containing the serialized JSON.
 
-**Sent messages** will be checked if the MQTT payload contains a valid JSON-encoded envelope:
+
+#### JSON downlink to instruct a node to send a message
+You can also send a JSON message to the topic `msh/2/json/mqtt/` to instruct a gateway node to send a message to the mesh. Note that the channel **must** be called "mqtt". The JSON message should contain the following fields:
 
 ```json
 {
-   "sender": "SENDER",
+   "from": <node number>,
+   "type": "type",
    "payload": {
        "key":"value"
        ...
@@ -108,7 +111,10 @@ If the message received contains valid JSON in the payload, the JSON is deserial
 }
 ```
 
-`sender` and `payload` fields are required for a valid envelope. If a valid MQTT message is found, the message is sent over the radio as a message of type `TEXT_MESSAGE_APP` with the serialized `payload` value in the message payload.
+`from` and `payload` fields are required for a valid envelope (note that in firmware <2.2.20 a field `sender` was required, but this is no longer the case). The `from` field should be equal to the node number of the node that will transmit the message. Optionally, you can specify a destination by setting the `to` field to the node number of the destination. If the `to` field is not set, the message will be broadcast to all nodes on the mesh.
+
+Currently two types of messages are supported: `"sendtext"` and `"sendposition"`.
+For the type `sendtext`, the `payload` should be a string containing the text to send. For the type `sendposition`, the payload should be an object with the fields `latitude_i`, `longitude_i`, `altitude` (optional) and `time` (optional).
 
 ### Basic Configuration
 

docs/software/mqtt/nodered.mdx

@@ -54,13 +54,13 @@ The JSON output only publishes the following subset of the messages on a Meshtas
 > Protobufs are mesh native.
 
 #### 1. Using JSON-encoded messages
-Make sure that option *JSON Output Enabled* is set in MQTT module options.
+Make sure that option *JSON Output Enabled* is set in MQTT module options and your channel is called "mqtt".
 
 Below is a valid JSON envelope for information sent by MQTT to a device for broadcast onto the mesh.
 
 ```json
 {
-  "sender":"whatever you want to be the SENDER",
+  "from":<node number of the transmitter>,
   "type":"sendtext",
   "payload": text or a json object go here
 }
@@ -979,11 +979,11 @@ The flow is:
 ]
 ```
 
-Sending a position to a device for broadcast to the mesh is much easier with JSON. This introduces a new MQTT Service Envelope type: "sendposition". A valid MQTT envelope and message to broadcast lat, lon, altitude looks like this.
+Sending a position to a device for broadcast to the mesh is much easier with JSON. It requires a channel called "mqtt". This introduces a new MQTT Service Envelope type: "sendposition". A valid MQTT envelope and message to broadcast lat, lon, altitude (optional) and time (optional) looks like this:
 
 ```json
 {
-  "sender": "someSender",
+  "from": 2130636288,
   "type": "sendposition",
   "payload": {
     "latitude_i": 399600000,