[Transform] Upgrade Mode docs (#132652)

Create docs for Transform's upgrade mode

Relate #132510

Author: prwhelan

rest-api-spec/src/main/resources/rest-api-spec/api/transform.set_upgrade_mode.json

@@ -0,0 +1,33 @@
+{
+  "transform.set_upgrade_mode":{
+    "documentation":{
+      "url":"https://www.elastic.co/guide/en/elasticsearch/reference/current/transform-set-upgrade-mode.html",
+      "description":"Sets a cluster wide upgrade_mode setting that prepares transform indices for an upgrade."
+    },
+    "stability":"stable",
+    "visibility":"public",
+    "headers":{
+      "accept": [ "application/json"]
+    },
+    "url":{
+      "paths":[
+        {
+          "path":"/_transform/set_upgrade_mode",
+          "methods":[
+            "POST"
+          ]
+        }
+      ]
+    },
+    "params":{
+      "enabled":{
+        "type":"boolean",
+        "description":"Whether to enable upgrade_mode Transform setting or not. Defaults to false."
+      },
+      "timeout":{
+        "type":"time",
+        "description":"Controls the time to wait before action times out. Defaults to 30 seconds"
+      }
+    }
+  }
+}