ingest-node.asciidoc - Clarify json processor (#21876)

Add examples for the json processor.

docs/reference/ingest/ingest-node.asciidoc

@@ -1510,15 +1510,72 @@ Converts a JSON string into a structured JSON object.
 | `add_to_root`  | no        | false    | Flag that forces the serialized json to be injected into the top level of the document. `target_field` must not be set when this option is chosen.
 |======
 
+Suppose you provide this configuration of the `json` processor:
+
+[source,js]
+--------------------------------------------------
+{
+  "json" : {
+    "field" : "string_source",
+    "target_field" : "json_target"
+  }
+}
+--------------------------------------------------
+
+If the following document is processed:
+
+[source,js]
+--------------------------------------------------
+{
+  "string_source": "{\"foo\": 2000}"
+}
+--------------------------------------------------
+
+after the `json` processor operates on it, it will look like:
+
 [source,js]
 --------------------------------------------------
 {
-  "json": {
-    "field": "{\"foo\": 2000}"
+  "string_source": "{\"foo\": 2000}",
+  "json_target": {
+    "foo": 2000
   }
 }
 --------------------------------------------------
 
+If the following configuration is provided, omitting the optional `target_field` setting:
+[source,js]
+--------------------------------------------------
+{
+  "json" : {
+    "field" : "source_and_target"
+  }
+}
+--------------------------------------------------
+
+then after the `json` processor operates on this document:
+
+[source,js]
+--------------------------------------------------
+{
+  "source_and_target": "{\"foo\": 2000}"
+}
+--------------------------------------------------
+
+it will look like:
+
+[source,js]
+--------------------------------------------------
+{
+  "source_and_target": {
+    "foo": 2000
+  }
+}
+--------------------------------------------------
+
+This illustrates that, unless it is explicitly named in the processor configuration, the `target_field` 
+is the same field provided in the required `field` configuration.
+
 [[kv-processor]]
 === KV Processor
 This processor helps automatically parse messages (or specific event fields) which are of the foo=bar variety.