Add a non-normative comment to clarify MXR behavior for pointer masking. (riscv#1667)

Author: martinmaas

src/zpm.adoc

@@ -148,6 +148,11 @@ It is worth noting that pointer masking is not applied to `SFENCE.\*`, `HFENCE.*
 
 MPRV and SPVP affect pointer masking as well, causing the pointer masking settings of the effective privilege mode to be applied. When MXR is in effect at the effective privilege mode where explicit memory access is performed, pointer masking does not apply.
 
+[NOTE]
+====
+Note that this includes cases where page-based virtual memory is not in effect; i.e., although MXR has no effect on permissions checks when page-based virtual memory is not in effect, it is still used in determining whether or not pointer masking should be applied.
+====
+
 [NOTE]
 ====
 Cache Management Operations (CMOs) must respect and take into account pointer masking. Otherwise, a few serious security problems can appear, including: