Add code documentation

base/ltcmd.dtx

@@ -4040,7 +4040,12 @@
 %   with a search for |=| signs. We need an \enquote{action} loop here
 %   so we do not get mislead by for example |{=}|. As the code here is for
 %   very much predictable types of input, we hard-code what constitutes
-%   math mode opening and closing.
+%   math mode opening and closing.  At the very beginning, the default
+%   key (|#1|) and the argument as given by the user (|#2|) are placed
+%   right after the \cs{q_recursion_stop}, so that when the recursion
+%   ends, the macros \cs{@@_arg_to_keyvalue_set_default:nn} or
+%   \cs{@@_arg_to_keyvalue_set_keyvalue:nn} can be used to grab these
+%   two items and set the \cs{ProcessedArgument} accordingly.
 %    \begin{macrocode}
 \cs_new_protected:Npn \@@_arg_to_keyvalue_auxv:nn #1#2
   {