doc: update documentation about predicate to query mapping

[fogfish]

consider the use-case:

t(Urn,L) :- f(Urn, "urn:rdfs:label", L), L = "Lomonosov Russia".

predicate f bind second variable to "urn:refs:label" but variable name unknown. The parser builds

#{t => [[urn,l],
   #{'@' => f,
     '_' => [urn,<<"urn:rdfs:label">>,l],
     l => <<"Lomonosov Russia">>}]}

The native query builder shall be aware of [urn,<<"urn:rdfs:label">>,l] mapping.