{azure-rest-api-specs} Add possible values for DomainValidationMethod (Azure#18469)

* As discussed with Elle Tojaroon, making the changes to include possible options for the domainValidationMethod property. with the following values
public const string CNameValidationValue = "cname-delegation";
public const string HttpTokenValidationValue = "http-token";

* Updating just the description of the domainValidationMethod with the possible values 'CNameValidationValue', 'HttpTokenValidationValue'

Author: RakeshMohanMSFT

specification/web/resource-manager/Microsoft.Web/stable/2021-01-15/Certificates.json

@@ -441,7 +441,7 @@
               "type": "string"
             },
             "domainValidationMethod": {
-              "description": "Method of domain validation for free cert",
+              "description": "Method of domain validation for free cert. Possible values include: 'CNameValidationValue', 'HttpTokenValidationValue'",
               "type": "string"
             }
           },