Document HiLo and perf aspects of database-generated keys

In the [SQL Server value generation page](https://docs.microsoft.com/en-us/ef/core/providers/sql-server/value-generation), but also add a section in [Efficient querying](https://docs.microsoft.com/en-us/ef/core/performance/efficient-querying#load-related-entities-eagerly-when-possible) that discusses the extra roundtrips and how to avoid them (HiLo or GUIDs).