Document support for configurable HTTP request factory in proxy clients

Author: jdreesen

Resources/doc/features/invalidation.rst

@@ -66,7 +66,8 @@ To refresh paths and routes, you can use ``refreshPath($path, $headers)`` and
 
     If you want to add a header (such as ``Authorization``) to *all*
     invalidation requests, you can use a
-    :ref:`custom HTTP client <custom HTTP client>` instead.
+    :ref:`custom HTTP client <custom HTTP client>` or
+    :ref:`custom HTTP request factory <custom HTTP request factory>` instead.
 
 .. _invalidation configuration:
 

Resources/doc/reference/configuration/proxy-client.rst

@@ -402,6 +402,15 @@ example to send a basic authentication header with each request, you can
 configure a service for the ``HttpClient`` and specify that in the
 ``http_client`` option of any of the cache proxy clients.
 
+.. _custom HTTP request factory:
+
+Custom HTTP Request Factory
+------------------
+
+The proxy client uses an implementation of ``Http\Message\RequestFactory`` to create HTTP requests.
+If you need to customize the request creation, you can configure your custom service and
+specify that in the ``request_factory`` option of any of the cache proxy clients.
+
 Caching Proxy Configuration
 ---------------------------