Add some more comments about the mechanism that substitutes dates inside the .mod file

src/DynareFlex.ll

@@ -258,6 +258,9 @@ DATE -?[0-9]+([ya]|m([1-9]|1[0-2])|q[1-4])
 
  /* Inside  of a Dynare statement */
 <DYNARE_STATEMENT>{DATE} {
+                           /* If a date is found within a statement, substitute it with a call to
+                              the dates() constructor in the input character stream. Then it will
+                              be handled by the rule that follows the present one. */
                            char* yycopy = strdup(yytext);
                            char* uput = yycopy + yyleng;
                            unput(')');

src/Statement.cc

@@ -50,6 +50,11 @@ NativeStatement::writeOutput(ostream& output, [[maybe_unused]] const string& bas
      boost::xpressive does not look for the longest match in an alternation, but stops at the first
      match from left to right. */
   string date_regex = R"((-?\d+([YyAa]|[Mm](1[0-2]|[1-9])|[Qq][1-4]|[SsHh][1-2])))";
+  /* NB: the following dance around the dollar sign (exclude it from lookbehind, then use it in a
+     temporary string after the first replace, then remove it in the second replace) has a purpose:
+     it allows the user to disable the substitution mechanism. For example, if the user writes
+     “$2024Q4” in a native statement, it will be transformed into “2024Q4” and not
+     “$dates('2024Q4')”. */
   sregex regex_lookbehind = sregex::compile(R"((?<!\$|\d|[a-zA-Z_]|-|'))" + date_regex);
   sregex regex_dollar = sregex::compile(R"((\$))" + date_regex);