Fix Nuxeo MongoDB documentation to take the MongoDB-EXT addon into account

At https://doc.nuxeo.com/810/nxdoc/mongodb/ , it is said:

You must keep the template corresponding to your SQL database in nuxeo.templates, because the SQL database is still used for other things (directories, audit, etc.).

This does not leave much room for
https://connect.nuxeo.com/nuxeo/site/marketplace/package/nuxeo-mongodb-ext?version=1.0.2
for which the aim is to NOT keep the template corresponding to the SQL database

I thus find the doc misleading.

It is said almost the same thing in the FT doc (For older versions of Nuxeo, ...), which should be revised in the same way than LTS 2016.