Update l3build.dtx

l3build.dtx

@@ -1691,6 +1691,10 @@
 %     \bottomrule
 %     \end{tabular}
 %   \end{center}
+%   The Lua tables mentionned here are in fact arrays of strings.
+%   From the |build.lua| file, one can modify the string and boolean values,
+%   add or remove entries in arrays. But it is not recommended to affect a
+%   whole new Lua table to |options| nor to its array entries.
 % \end{variable}
 %
 % \subsection{Utility functions}