[doc] Mention to not add ~beta when using set-version script

doc/release/branch.tex

@@ -75,6 +75,10 @@ On the new \texttt{stable} branch, execute the script:
 \begin{verbatim}
 ./dev/set-version.sh NN.M # to be replaced with actual major/minor version
 \end{verbatim}
+
+\textbf{Do not add \texttt{\textasciitilde{}beta} here, this will be done
+manually in section~\ref{sec:validation:version}.}
+
 This will:
 \begin{itemize}
   \item update the \texttt{Changelog}s

doc/release/validation.tex

@@ -28,6 +28,7 @@ If you don't know them, ask plug-in developers to verify them.
 \end{itemize}
 
 \section{Fix version}
+\label{sec:validation:version}
 
 The tasks listed in this section are performed by the Release Manager.