Update ngsi_sth_sink.md

doc/cygnus-ngsi/flume_extensions_catalogue/ngsi_sth_sink.md

@@ -299,6 +299,9 @@ Since version 3.0.0 of cygnus index named `cyg_agg_opt` is created in this way b
 | dm-by-entity       | attrName, resolution, origin                       |
 | dm-by-attribute    | resolution, origin                                 |
 
+
+Note that attempting to use any other datamodel different from these ones will cause a "Unknown data model" ERROR in log traces. In addition, note that default datamodel for STH/Mongo sink is dm-by-entity as all others sinks which uses datamodel.
+
 ## <a name="section2"></a>Administration guide
 ### <a name="section2.1"></a>Configuration
 `NGSISTHSink` is configured through the following parameters: