Query: add better documentation explaining how to use instancePropagatesNullability and argumentsPropagateNullability on SqlFunctionExpression

It is an advanced scenario and it's documented here: https://learn.microsoft.com/en-us/ef/core/querying/user-defined-function-mapping#configuring-nullability-of-user-defined-function-based-on-its-arguments
but it's also quite complicated and nuanced - we should add some extra contexts (as part of the xml documentation) that would explain how to use it properly, and if user is not certain, how to use it safely (i.e. in a way that will not produce incorrect sql)