[docs] SMTPHandler secure argument without credentials

logging.handlers.SMTPHandler only uses the 'secure' argument when credentials are specified. https://docs.python.org/3/library/logging.handlers.html#logging.handlers.SMT... says:

"To specify the use of a secure protocol (TLS), pass in a tuple to the secure argument. This will only be used when authentication credentials are supplied."

The docstring confirms this, but I can't find an explanation of the intent behind this behaviour. Perhaps it would be a good idea to describe this as well.