Add note about AUIPC+JALR range in RV64

src/rv64.tex

@@ -146,6 +146,12 @@ \subsubsection*{Integer Register-Immediate Instructions}
 adds it to the address of the AUIPC instruction,
 then places the result in register {\em rd}.
 
+\begin{commentary}
+Note that the set of addresses or offsets that can be formed by pairing LUI
+with LD, AUIPC with JALR, etc.\@ in RV64 includes all signed 32-bit offsets
+except for the range [$2^{31}{-}2^{11}$, $2^{31}{-}1$].
+\end{commentary}
+
 \subsubsection*{Integer Register-Register Operations}
 
 \vspace{-0.2in}