Merge pull request #12923 from maxunbearable/task/rest-md-files

Added md help files for REST connector

ui-ngx/src/assets/help/en_US/widget/lib/gateway/rest-json_fn.md

@@ -0,0 +1,50 @@
+### Expressions
+#### JSON Path
+
+The expression field is used to specify the path for extracting data from an incoming HTTP request body.
+
+JSONPath expressions specifies the items within a JSON structure (which could be an object, array, or nested combination of both) that you want to access.  
+These expressions can select elements from JSON data on specific criteria.  
+Basic overview of how JSONPath expressions are structured:
+
+- `$`: The root element of the JSON document;
+- `.`: Child operator used to select child elements. For example, $.store.book ;
+- `[]`: Child operator used to select child elements. $['store']['book'] accesses the book array within a store object;
+
+#### Examples
+
+Incoming request body:
+
+```json
+{
+	"sensorModelInfo": {
+		"sensorName": "AM-123",
+		"sensorType": "myDeviceType"
+	},
+	"data": {
+		"temp": 12.2,
+		"hum": 56,
+		"status": "ok"
+	},
+  "rxInfo": [
+    {
+      "rssi": 50,
+      "snr": 3
+    },
+    {
+      "rssi": 22,
+      "snr": 1
+    }
+  ]
+}
+```
+<br>
+
+Examples below shows on how to extract values from incoming request body.
+
+| JSON Path Expression            | Extracted data                              |
+|---------------------------------|---------------------------------------------|
+| `${sensorModelInfo.sensorName}` | `AM-123`                                    |
+| `${data}`                       | `{"temp": 12.2, "hum": 56, "status": "ok"}` |
+| `${$.data.temp}`                | `12.2`                                      |
+| `${$.rxInfo[0].rssi}`           | `50`                                        |