minor #18481 [HttpClient] Add a note about requirement to use URI templates (clement-michelet)

This PR was merged into the 6.3 branch.

Discussion
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[HttpClient] Add a note about requirement to use URI templates

Closes #18480

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Commits
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40c513d [HttpClient] Add a note about requirement to use URI templates

Author: javiereguiluz

http_client.rst

@@ -946,6 +946,11 @@ If you want to define your own logic to handle variables of URI templates, you
 can do so by redefining the ``http_client.uri_template_expander`` alias. Your
 service must be invokable.
 
+.. note::
+
+    Support for URI template requires a vendor or to pass your own expander
+    ``\Closure`` implementation to expand the URI.
+
 .. versionadded:: 6.3
 
     The :class:`Symfony\\Component\\HttpClient\\UriTemplateHttpClient` was