Add a comment explaining the fix

Author: pablogsal

Lib/test/test_asyncio/test_sslproto.py

@@ -429,6 +429,10 @@ def test_start_tls_server_1(self):
 
         server_context = test_utils.simple_server_sslcontext()
         client_context = test_utils.simple_client_sslcontext()
+        # bpo-35031: Some FreeBSD buildbots fail to run this test
+        # as the eof was not being received by the server if the payload
+        # size is not big enough. This behaviour only appears if the
+        # client is using TLS1.3.
         client_context.options |= ssl.OP_NO_TLSv1_3
 
         def client(sock, addr):