Add comment for broken_pipe_warning

Author: kotfu

cmd2/cmd2.py

@@ -290,6 +290,7 @@ def __init__(self, completekey: str = 'tab', stdin=None, stdout=None, *,
         # The error that prints when a non-existent command is run
         self.default_error = "{} is not a recognized command, alias, or macro"
 
+        # If non-empty, this contents will be displayed if a broken pipe error occurs
         self.broken_pipe_warning = ''
 
         # Commands that will run at the beginning of the command loop