Documentation

I was going through the documentation to set up a mypy.ini for a new project and noticed that show_error_codes and show_error_code_links are missing here: https://mypy.readthedocs.io/en/stable/config_file.html#configuring-error-messages

The fact that error codes are shown by default is mentioned in the command line documentation (https://mypy.readthedocs.io/en/stable/command_line.html#configuring-error-messages) though --show-error-code-links is missing.