Update description for property eTag (#22230)

* Update description for property eTag

* Switch to If-Match header approach

Author: pednekarpraj

specification/cost-management/resource-manager/Microsoft.CostManagement/stable/2022-10-01/examples/scheduledActions/scheduledAction-createOrUpdate-private.json

@@ -2,6 +2,7 @@
   "parameters": {
     "api-version": "2022-10-01",
     "name": "monthlyCostByResource",
+    "If-Match": "",
     "scheduledAction": {
       "kind": "Email",
       "properties": {

specification/cost-management/resource-manager/Microsoft.CostManagement/stable/2022-10-01/examples/scheduledActions/scheduledAction-createOrUpdate-shared.json

@@ -3,6 +3,7 @@
     "api-version": "2022-10-01",
     "scope": "subscriptions/00000000-0000-0000-0000-000000000000",
     "name": "monthlyCostByResource",
+    "If-Match": "",
     "scheduledAction": {
       "kind": "Email",
       "properties": {

specification/cost-management/resource-manager/Microsoft.CostManagement/stable/2022-10-01/examples/scheduledActions/scheduledAction-insightAlert-createOrUpdate-shared.json

@@ -3,6 +3,7 @@
     "api-version": "2022-10-01",
     "scope": "subscriptions/00000000-0000-0000-0000-000000000000",
     "name": "dailyAnomalyByResource",
+    "If-Match": "",
     "scheduledAction": {
       "kind": "InsightAlert",
       "properties": {

specification/cost-management/resource-manager/Microsoft.CostManagement/stable/2022-10-01/scheduledActions.json

@@ -161,6 +161,9 @@
             "schema": {
               "$ref": "#/definitions/ScheduledAction"
             }
+          },
+          {
+            "$ref": "#/parameters/IfMatchOptionalParameter"
           }
         ],
         "responses": {
@@ -292,6 +295,9 @@
             "schema": {
               "$ref": "#/definitions/ScheduledAction"
             }
+          },
+          {
+            "$ref": "#/parameters/IfMatchOptionalParameter"
           }
         ],
         "responses": {
@@ -649,7 +655,7 @@
         "eTag": {
           "type": "string",
           "readOnly": true,
-          "description": "Resource Etag."
+          "description": "Resource Etag. For update calls, eTag is mandatory. Fetch the resource's eTag by doing a 'GET' call first and then including the latest eTag as part of the request body or 'If-Match' header while performing the update. For create calls, eTag is not required."
         },
         "kind": {
           "$ref": "#/definitions/ScheduledActionKind",
@@ -913,6 +919,13 @@
       "required": true,
       "type": "string",
       "x-ms-parameter-location": "method"
+    },
+    "IfMatchOptionalParameter": {
+      "name": "If-Match",
+      "in": "header",
+      "required": false,
+      "description": "ETag of the Entity. Not required when creating an entity, but required when updating an entity.",
+      "type": "string"
     }
   }
 }