explain that ̀ file=None` will print the message in stdout

Author: tobiasdiez

src/sage/doctest/forker.py

@@ -261,7 +261,8 @@ def showwarning_with_traceback(message, category, filename, lineno, file=None, l
     r"""
     Displays a warning message with a traceback.
 
-    INPUT: see :func:`warnings.showwarning`.
+    INPUT: see :func:`warnings.showwarning` with the difference that with ``file=None``
+           the message will be written to stdout.
 
     OUTPUT: none