Re-apply #21565

Fixes an errant merge

Author: rwestMSFT

docs/relational-databases/tables/creating-a-system-versioned-temporal-table.md

@@ -142,9 +142,10 @@ GO
 ALTER TABLE InsurancePolicy
     ADD
         ValidFrom DATETIME2 GENERATED ALWAYS AS ROW START HIDDEN
-            CONSTRAINT DF_SysStart DEFAULT SYSUTCDATETIME()
+            CONSTRAINT DF_InsurancePolicy_ValidFrom DEFAULT SYSUTCDATETIME()
       , ValidTo DATETIME2 GENERATED ALWAYS AS ROW END HIDDEN
-            CONSTRAINT DF_SysEnd DEFAULT CONVERT(DATETIME2, '9999-12-31 23:59:59.9999999'),
+            CONSTRAINT DF_InsurancePolicy_ValidTo DEFAULT CONVERT(DATETIME2, '9999-12-31 23:59:59.9999999')
+      , PERIOD FOR SYSTEM_TIME (ValidFrom, ValidTo);
 GO
 
 ALTER TABLE InsurancePolicy