AR: Comment itrigger limited to 32 ints if XLEN=32

Resolves #904.

xml/hwbp_registers.xml

@@ -1141,6 +1141,13 @@
         In addition the trigger can be enabled for non-maskable interrupts using
         \FcsrItriggerNmi.
 
+        \begin{commentary}
+        If XLEN is 32, then it is not possible to set a trigger on every
+        standard interrupt, because the RISC-V Privileged Specification reserves
+        numbers 32 through 47 and some of those numbers are already being used
+        by the RISC-V Advanced Interrupt Architecture.
+        \end{commentary}
+
         Hardware may only support a subset of interrupts for this trigger.  A
         debugger must read back \RcsrTdataTwo after writing it to confirm the
         requested functionality is actually supported.