minor #19192 Adding precision on how to define attributes on the root node for the XML Encoder (ktherage)

This PR was submitted for the 6.3 branch but it was squashed and merged into the 5.4 branch instead.

Discussion
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Adding precision on how to define attributes on the root node for the XML Encoder

This pull request aims to clarify a point that was not clearly exposed in the documentation when trying to define attributes on the root node for XML.

Commits
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c09286e Adding precision on how to define attributes on the root node for the XML Encoder

Author: OskarStark

components/serializer.rst

@@ -1162,6 +1162,23 @@ the key  ``#comment`` can be used for encoding XML comments::
 You can pass the context key ``as_collection`` in order to have the results
 always as a collection.
 
+.. note::
+
+    You may need to add some attributes on the root node::
+
+        $encoder = new XmlEncoder();
+        $encoder->encode([
+            '@attribute1' => 'foo',
+            '@attribute2' => 'bar',
+            '#' => ['foo' => ['@bar' => 'value', '#' => 'baz']]
+        ], 'xml');
+
+        // will return:
+        // <?xml version="1.0"?>
+        // <response attribute1="foo" attribute2="bar">
+        // <foo bar="value">baz</foo>
+        // </response>
+
 .. tip::
 
     XML comments are ignored by default when decoding contents, but this