doc: add documentation for response.flushHeaders()

PR-URL: #28807
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

doc/api/http.md

@@ -614,7 +614,7 @@ request was initiated via [`http.get()`][].
 added: v1.6.0
 -->
 
-Flush the request headers.
+Flushes the request headers.
 
 For efficiency reasons, Node.js normally buffers the request headers until
 `request.end()` is called or the first chunk of request data is written. It
@@ -1183,6 +1183,13 @@ added: v0.0.2
 The `response.finished` property will be `true` if [`response.end()`][]
 has been called.
 
+### response.flushHeaders()
+<!-- YAML
+added: v1.6.0
+-->
+
+Flushes the response headers. See also: [`request.flushHeaders()`][].
+
 ### response.getHeader(name)
 <!-- YAML
 added: v0.4.0
@@ -2206,6 +2213,7 @@ not abort the request or do anything besides add a `'timeout'` event.
 [`new URL()`]: url.html#url_constructor_new_url_input_base
 [`removeHeader(name)`]: #http_request_removeheader_name
 [`request.end()`]: #http_request_end_data_encoding_callback
+[`request.flushHeaders()`]: #http_request_flushheaders
 [`request.getHeader()`]: #http_request_getheader_name
 [`request.setHeader()`]: #http_request_setheader_name_value
 [`request.setTimeout()`]: #http_request_settimeout_timeout_callback