Default allowed file extensions help text is inaccurate; comma-separation is not allowed

The form generated by file_entity_settings_form() has help text for the Default allowed file extensions (file_entity_default_allowed_extensions) field that's copied from core's help text on file field configuration. It reads: "Separate extensions with a space or comma and do not include the leading dot."

Unfortunately this is not accurate. Core's implementation adds a validator (_file_generic_settings_extensions) that cleans up comma-separated values before saving to the db, but File Entity does not. I'd like to suggest we simply update the help text to be accurate.