Document special handling for "&" in SimpleXml::addChild()

See the comments section for that page: https://www.php.net/manual/en/simplexmlelement.addchild.php

Author: cweiske

reference/simplexml/simplexmlelement/addChild.xml

@@ -38,6 +38,10 @@
       <para>
        If specified, the value of the child element.
       </para>
+      <para>
+       Special characters <literal>&lt;</literal> and <literal>&gt;</literal> get escaped
+       automatically, <literal>&amp;</literal> must be escaped manually.
+      </para>
      </listitem>
     </varlistentry>
     <varlistentry>