something like this perhaps

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" 
version="1.0">

<xsl:key name="d" match="document" use="@id"/>

   <xsl:template match="/">
     <people>
       <xsl:copy-of select="data/persons/person[document[not(key('d',@relid))]]/name"/>
     </people>
   </xsl:template>

</xsl:stylesheet>