F-11025: update_flash: document deliberately inert PCR extend block

The PCR-extension block in wolfBoot_unlock_disk() is guarded with
!defined(ARCH_SIM) while the function itself only builds for
ARCH_SIM, so it can never compile. The exclusion is deliberate
(eb2978ab: do not extend the unseal PCR on the simulator, or the
secret becomes un-unsealable), not an oversight.

Clarify the comment with the intended build scope instead of
removing the block: the code and the WOLFBOOT_NO_UNSEAL_PCR_EXTEND
option exist for the day the unlock-disk path is ported to a
non-sim target.

src/update_flash.c

@@ -1499,7 +1499,11 @@ int wolfBoot_unlock_disk(void)
         /* TODO: Unlock disk */
 
 
-        /* Extend a PCR from the mask to prevent future unsealing */
+        /* Extend a PCR from the mask to prevent future unsealing.
+         * Non-sim only: extending on the simulator would lock the
+         * PCR and block future unseals (eb2978ab). The function is
+         * ARCH_SIM-only today, so the block is inert until the
+         * unlock path is ported. */
     #if !defined(ARCH_SIM) && !defined(WOLFBOOT_NO_UNSEAL_PCR_EXTEND)
         {
         uint32_t pcrMask;