[doc] release manual

- update what the set-version script does

doc/release/branch.tex

@@ -79,6 +79,7 @@ This will:
   \item update the \texttt{VERSION} and \texttt{VERSION\_CODENAME} files
   \item update the \texttt{opam} files (Frama-C, lint, hdrck)
   \item update the API doc
+  \item update the Frama-C build script
 \end{itemize}
 
 Merge the \texttt{stable} branch in the \texttt{master} branch.