Fixed an issue, where the output G-code file name was not always updated

from the current Model/ModelObjects.
Fixed a possible race condition in updating Print::m_placeholder_parser
with the proposed filename / filename base.
Improved documentation (source code comments).

src/libslic3r/GCode.cpp

@@ -717,6 +717,7 @@ void GCode::_do_export(Print &print, FILE *file)
     // Prepare the helper object for replacing placeholders in custom G-code and output filename.
     m_placeholder_parser = print.placeholder_parser();
     m_placeholder_parser.update_timestamp();
+    print.update_object_placeholders(m_placeholder_parser.config_writable());
 
     // Get optimal tool ordering to minimize tool switches of a multi-exruder print.
     // For a print by objects, find the 1st printing object.