Add note about PTW (#553)

Fixes #552

---------

Signed-off-by: Tariq Kurd <tariqandlaura@gmail.com>
Co-authored-by: Alexander Richardson <mail@alexrichardson.me>

src/cheri-pte-ext.adoc

@@ -26,6 +26,8 @@ NOTE: The description below assumes that {cheri_pte_ext_name} has been implement
 
 The minimum level of PTE support is to set CW to 1 in all PTEs intended for storing capabilities (i.e. private anonymous mappings) and leave <<sstatusreg_pte,sstatus>>.UCRG and CRG in all PTEs set to 0, which will allow capabilities with their tags set to be loaded and stored successfully.
 
+NOTE: Hardware initiated memory accesses from the page-table walker are not checked by a capability.
+
 [#cheri_pte_fault]
 === CHERI PTE page faults