Refine URT_040 requirement (ARC #201)

Add non-normative text to indicate why redundant requirement exists.

Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>

uefi.adoc

@@ -80,7 +80,8 @@ See additional <<uefi-rt, requirements for UEFI runtime services>>.
              * `GetTime()` and `SetTime()` MUST be appropriately described in the `EFI_RT_PROPERTIES_TABLE`.
 | `URT_030` a| The UEFI `ResetSystem()` runtime service MUST be implemented.
 2+| _The OS MUST call the `ResetSystem()` runtime service call to reset or shutdown the system, preferring this to SBI, ACPI or other system-specific mechanisms. This allows for systems to perform any required system tasks on the way out (e.g. servicing `UpdateCapsule()` or persisting non-volatile variables in some systems)._
-| `URT_040` | The UEFI variables MUST persist across calls to the `ResetSystem()` runtime service call.
+| `URT_040` | The non-volatile UEFI variables MUST persist across calls to the `ResetSystem()` runtime service call.
+2+| _This rule is included in this specification to address a common mistake in implementing the UEFI requirements for non-volatile variables, even though it may appear redundant with the existing UEFI specification._
 | `URT_050` | UEFI runtime services MUST be able to update the UEFI variables directly without the aid of an OS.
 2+| _UEFI variables are normally saved in a dedicated storage which is not directly accessible by the operating system._
 | `URT_060` a| The following requirements MUST be met for systems with UEFI secure boot: