Documents new timeout protocol field

docs/internals/protocol.rst

@@ -54,6 +54,13 @@ Message format
     will be expired when the message is received and the expiration date
     has been exceeded.
 
+* timeouts
+    :`tuple`:
+
+    .. versionadded:: 2.7
+
+    Task execution timeouts. This is a tuple of hard and soft timeouts.
+    Timeout values are `int` or `float`.
 
 Extensions
 ==========