[ML] Add REST spec for the update data frame analytics endpoint (#59253) (#59281)

Closes #59148

Backport of #59253

x-pack/plugin/src/test/resources/rest-api-spec/api/ml.update_data_frame_analytics.json

@@ -0,0 +1,29 @@
+{
+  "ml.update_data_frame_analytics":{
+    "documentation":{
+      "url":"https://www.elastic.co/guide/en/elasticsearch/reference/current/update-dfanalytics.html",
+      "description":"Updates certain properties of a data frame analytics job."
+    },
+    "stability":"experimental",
+    "url":{
+      "paths":[
+        {
+          "path":"/_ml/data_frame/analytics/{id}/_update",
+          "methods":[
+            "POST"
+          ],
+          "parts":{
+            "id":{
+              "type":"string",
+              "description":"The ID of the data frame analytics to update"
+            }
+          }
+        }
+      ]
+    },
+    "body":{
+      "description":"The data frame analytics settings to update",
+      "required":true
+    }
+  }
+}

x-pack/plugin/src/test/resources/rest-api-spec/test/ml/data_frame_analytics_crud.yml

@@ -2100,3 +2100,53 @@ setup:
               "includes": ["excluded"]
             }
           }
+
+---
+"Test update given all updatable settings":
+
+  - do:
+      ml.put_data_frame_analytics:
+        id: "update-test-job"
+        body: >
+          {
+            "source": {
+              "index": "index-source"
+            },
+            "dest": {
+              "index": "index-dest"
+            },
+            "analysis": {"outlier_detection":{}},
+            "description": "before update",
+            "model_memory_limit": "20mb",
+            "allow_lazy_start": false
+          }
+  - match: { id: "update-test-job" }
+  - match: { description: "before update" }
+  - match: { model_memory_limit: "20mb" }
+  - match: { allow_lazy_start: false }
+
+  - do:
+      ml.update_data_frame_analytics:
+        id: "update-test-job"
+        body: >
+          {
+            "description": "after update",
+            "model_memory_limit": "30mb",
+            "allow_lazy_start": true
+          }
+  - match: { id: "update-test-job" }
+  - match: { description: "after update" }
+  - match: { model_memory_limit: "30mb" }
+  - match: { allow_lazy_start: true }
+
+---
+"Test update given missing analytics":
+
+  - do:
+      catch: missing
+      ml.update_data_frame_analytics:
+        id: "missing-analytics"
+        body: >
+          {
+            "description": "blah"
+          }