[ReleasePR reservations] Swagger correctness fixes (#1835)

Create to sync Azure/azure-rest-api-specs#15520
[ReCreate this PR](https://github.com/azure-resource-manager-schemas/compare/main...AzureSDKAutomation:sdkAuto/reservations?expand=1)

Author: msftbot[bot]

schemas/2019-04-01/Microsoft.Capacity.json

@@ -112,6 +112,20 @@
           "type": "string",
           "description": "Friendly name of the Reservation"
         },
+        "instanceFlexibility": {
+          "oneOf": [
+            {
+              "type": "string",
+              "enum": [
+                "On",
+                "Off"
+              ]
+            },
+            {
+              "$ref": "https://schema.management.azure.com/schemas/common/definitions.json#/definitions/expression"
+            }
+          ]
+        },
         "quantity": {
           "oneOf": [
             {