[ReleasePR web] {azure-rest-api-specs} Add possible values for DomainValidationMethod (#2276)

Create to sync Azure/azure-rest-api-specs#18469
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Author: msftbot[bot]

schemas/2021-01-15/Microsoft.Web.json

@@ -5322,7 +5322,7 @@
         },
         "domainValidationMethod": {
           "type": "string",
-          "description": "Method of domain validation for free cert"
+          "description": "Method of domain validation for free cert. Possible values include: 'CNameValidationValue', 'HttpTokenValidationValue'"
         },
         "hostNames": {
           "oneOf": [