Add note about the nonsmall-order check on rk.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

protocol/protocol.tex

@@ -3893,6 +3893,9 @@ where
         using $\AuthSignRandomizedPublic$ as the public key ---
         i.e.\ $\SpendAuthSigVerify{\AuthSignRandomizedPublic}(\SigHash, \spendAuthSig) = 1$.
 \end{consensusrules}
+
+\nnote{The check that $\AuthSignRandomizedPublic$ is not of small order is technically redundant with
+a check in the \spendCircuit, but it is simple and cheap to also check this outside the circuit.}
 } %sapling
 
 
@@ -9536,6 +9539,16 @@ Peter Newell's illustration of the Jubjub bird, from \cite{Carroll1902}.
 \intropart
 \section{Change History}
 
+\subparagraph{2018.0-beta-22}
+
+\begin{itemize}
+    \item No changes to \Sprout.
+\sapling{
+    \item Add a note about redundancy in the nonsmall-order checking of $\AuthSignRandomizedPublic$.
+} %sapling
+\end{itemize}
+
+\introlist
 \subparagraph{2018.0-beta-21}
 
 \begin{itemize}