Document that using JDBC repo, queryFilter work only on properties declared as searchable

In the integrator's guide, we explain how to build queryFilters (http://openidm.forgerock.org/doc/integrators-guide/index.html#constructing-queries). We should mention that the properties on which we do queries should be searchable for it to work when we use a JDBC repo.

(searchable concept is explained there: http://openidm.forgerock.org/doc/integrators-guide/index.html#searches-with-generic-mappings)