Add information about when to start sending response (#389)

specs/www.rst

@@ -165,6 +165,11 @@ Response Start - ``send`` event
 Sent by the application to start sending a response to the client. Needs to be
 followed by at least one response content message.
 
+Protocol servers *need not* flush the data generated by this event to the
+send buffer until the first *Response Body* event is processed.
+This may give them more leeway to replace the response with an error response
+in case internal errors occur while handling the request.
+
 You may send a ``Transfer-Encoding`` header in this message, but the server
 must ignore it. Servers handle ``Transfer-Encoding`` themselves, and may opt
 to use ``Transfer-Encoding: chunked`` if the application presents a response
@@ -199,7 +204,7 @@ Response Body - ``send`` event
 Continues sending a response to the client. Protocol servers must
 flush any data passed to them into the send buffer before returning from a
 send call. If ``more_body`` is set to ``False``, and the server is not
-expecting *Response Trailers* this will close the connection.
+expecting *Response Trailers* this will complete the response.
 
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