Write docs for cakephp/cakephp#17416

Author: markstory

en/appendices/5-1-upgrade-guide.rst

@@ -14,6 +14,9 @@ Http
 
 - ``SecurityHeadersMiddleware::setPermissionsPolicy()`` was added. This method
   adds the ability to define ``permissions-policy`` header values.
+- ``Client`` now emits ``HttpClient.beforeSend`` and ``HttpClient.afterSend``
+  events when requests are sent. You can use these events to perform logging,
+  caching or collect telemetry.
 
 Validation
 ----------

en/core-libraries/httpclient.rst

@@ -358,6 +358,17 @@ method::
     ]);
     $http->addCookie(new Cookie('session', 'abc123'));
 
+Client Events
+=============
+
+``Client`` will emit events when requests are sent. The
+``HttpClient.beforeSend`` event is fired before a request is sent, and
+``HttpClient.afterSend`` is fired after a request is sent. You can modify the
+request, or set a response in a ``beforeSend`` listener. The ``afterSend`` event
+is triggered for all requests, even those that have their responses set by
+a ``beforeSend`` event.
+
+
 .. _httpclient-response-objects:
 
 Response Objects