Docu: DM sections for STH sinks

[fgalan]

Postgis sink documentation has a section in which DM are described:

https://github.com/telefonicaid/fiware-cygnus/blob/master/doc/cygnus-ngsi/flume_extensions_catalogue/ngsi_postgresql_sink.md#section1.2

However, there isn't a similar section in STH sinks:

https://github.com/telefonicaid/fiware-cygnus/blob/master/doc/cygnus-ngsi/flume_extensions_catalogue/ngsi_sth_sink.md
https://github.com/telefonicaid/fiware-cygnus/blob/master/doc/cygnus-ngsi/flume_extensions_catalogue/ngsi_mongo_sink.md

So a similar table should be included, in this case described the MongoDB index created in each case.

See: #2205 (comment)

[AlvaroVega]

@fgalan should we close this ?

There is no table about DM in mongo/STH sink but there is tree entries that could be translated into table rows:
https://github.com/telefonicaid/fiware-cygnus/blob/master/doc/cygnus-ngsi/flume_extensions_catalogue/ngsi_sth_sink.md#mongodb-collections-naming-conventions

[fgalan]

I think the issue is still valid (although minor) and it would be solved as you said translating those entries into a table.

Documentation also should explain what happens when a DM different from these one is used in the sink configuration. It seems that in that case, the default DM is used, without raising any warning/error (maybe a warning should be raised...).