fixed exception in TXMLDataBindingItem.GetDocumentation

[Deeem2031]

see docwiki.embarcadero.com Xml.XMLIntf.IXMLNode.Text:

If the node has children (other than a single DOM text node), reading or setting Text causes an exception.

GetDocumentation fails on elements like this:

    <xs:element name="xy" type="xyType">
      <xs:annotation>
        <xs:documentation xml:lang="de">
          <translation>xy</translation>
          <description>
            <short>kurze Beschreibung</short>
          </description>
        </xs:documentation>
      </xs:annotation>
    </xs:element>

This solution simply ignores every documentation with more than a single child element.