Documentation: Add remark about using different ADO.NET providers for database X

[FransBouma]

It's perfectly possible to use whatever ADO.NET provider for a given supported database, but it's not that clear. (just state the provider name in the connection string definition). The documentation should have a remark about this to make it more clear you can e.g. use a 3rd party ADO.NET provider for Postgresql or Oracle instead of the official ones without changing any code:

just add providerName = '_provider invariant name_' right after the whole connectionString stuff.