Add explicit note about PTE bits in non-leaf nodes

Explicitly say that they are still reserved in non-leaf nodes. This matches the behaviour for Svpmbt for example (and I copied some of the text from there).

src/cheri-pte-ext.adoc

@@ -71,7 +71,9 @@ The page table entry format remains unchanged for Sv32. However, two new
 bits, Capability Write (CW) and Capability Read
 Generation (CRG), are added to leaf PTEs in Sv39, Sv48 and Sv57 as shown in
 xref:sv39pte[xrefstyle=short], xref:sv48pte[xrefstyle=short] and
-xref:sv57pte[xrefstyle=short] respectively.
+xref:sv57pte[xrefstyle=short] respectively. For non-leaf PTEs these bits
+remain reserved and must be cleared by software for forward compatibility,
+or else a page-fault exception is raised.
 
 .Sv39 page table entry
 [#sv39pte]