add doc

include/deal.II/base/parameter_handler.h

@@ -1137,10 +1137,14 @@ class ParameterHandler : public Subscriptor
    *
    * The action is executed in three different circumstances:
    * - With the default value of the parameter with name @p name, at
-   *   the end of the current function. This is useful because it allows
-   *   for the action to execute whatever it needs to do at least once
-   *   for each parameter, even those that are not actually specified in
-   *   the input file (and thus remain at their default values).
+   *   the end of the current function if @p execute_action is set to
+   *   true. This is useful because it allows for the action to execute
+   *   whatever it needs to do at least once for each parameter, even
+   *   those that are not actually specified in the input file (and
+   *   thus remain at their default values). Note that if the action
+   *   is executed, it converts the default value to a string and back
+   *   afterwards. This can lead to round-off errors so that the default
+   *   values might change in the case of floating-point numbers.
    * - Within the ParameterHandler::set() functions that explicitly
    *   set a value for a parameter.
    * - Within the parse_input() function and similar functions such

source/base/parameter_handler.cc

@@ -901,7 +901,6 @@ ParameterHandler::add_action(
 
   if (execute_action)
     {
-      // as documented, run the action on the default value at the very end
       const std::string default_value = entries->get<std::string>(
         get_current_full_path(entry) + path_separator + "default_value");
       action(default_value);