.Add one piece of documentation

siunitx-compound.dtx

@@ -165,6 +165,9 @@
 %   \begin{syntax}
 %     |compound-separator-mode| = |number|\verb"|"|text|
 %   \end{syntax}
+%   Choice which determines whether the separators between components
+%   of compound quantity are typeset a |number| or as |text|;
+%   the latter is the standard setting.
 % \end{function}
 %
 % \begin{function}{compound-units}