Add note about supporting a single lifetime action (Azure#16590)

Resolves Azure#15633 without introducing a breaking API change or tight coupling with current service behavior despite their OpenAPI definition.

Author: heaths

sdk/keyvault/Azure.Security.KeyVault.Certificates/src/CertificatePolicy.cs

@@ -226,6 +226,7 @@ public bool? CertificateTransparency
 
         /// <summary>
         /// Gets the actions to be executed at specified times in the certificates lifetime.
+        /// Currently, only a single <see cref="LifetimeAction"/> is allowed.
         /// </summary>
         public IList<LifetimeAction> LifetimeActions { get; } = new List<LifetimeAction>();