[DOCS] Upsert documentation clarification (#120684)

Co-authored-by: Liam Thompson <32779855+leemthompo@users.noreply.github.com>

docs/reference/docs/update.asciidoc

@@ -307,23 +307,19 @@ POST test/_update/1
 [discrete]
 ===== Upsert
 
-If the document does not already exist, the contents of the `upsert` element
-are inserted as a new document. If the document exists, the
-`script` is executed:
+An upsert operation lets you update an existing document or insert a new one if it doesn't exist, in a single request.
+
+In this example, if the product with ID `1` exists, its price will be updated to `100`. If the product does not exist, a new document with ID `1` and a price of `50` will be inserted.
 
 [source,console]
 ----
-POST test/_update/1
+POST /test/_update/1
 {
-  "script": {
-    "source": "ctx._source.counter += params.count",
-    "lang": "painless",
-    "params": {
-      "count": 4
-    }
+  "doc": {
+    "product_price": 100
   },
   "upsert": {
-    "counter": 1
+    "product_price": 50
   }
 }
 ----