Issue #20112: Improve BaseHTTPRequestHandler.error_message_format documentation

* Explain how 'error_message_format' is used by 'send_error' method
* Mention that 'responses' is used by 'send_response_only' and 'send_error' methods
* Replace 'class variables' with 'attributes'

Initial patch by Anastasia Filatova.
1 file changed