minor symfony#9249 Add example of setting the data of XML nodes (pyatnitsev, javiereguiluz)

This PR was submitted for the 4.0 branch but it was merged into the 3.4 branch instead (closes symfony#9249).

Discussion
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Add example of setting the data of XML nodes

Add an example serialization of node with attribute and value in same time

Commits
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7c862db Minor reword
d415361 fix minor syntax issue
51ba232 fix makeup
c4aecf4 add example

Author: javiereguiluz

components/serializer.rst

@@ -903,6 +903,18 @@ The array keys beginning with ``@`` are considered XML attributes::
     //     <foo bar="value" />
     // </response>
 
+Use the special ``#`` key to define the data of a node::
+
+    array('foo' => array('@bar' => 'value', '#' => 'baz'));
+
+    // is encoded as follows:
+    // <?xml version="1.0"?>
+    // <response>
+    //     <foo bar="value">
+    //        baz
+    //     </foo>
+    // </response>
+
 Context
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