Document dehavior around default value handling, .NET type defaults and sentinel values #5057
Description
An extremely common issue is users getting confused about EF's behavior around default values: they e.g. set a default true value for a bool property, and are then surprised by not being to insert false.
Our documentation on generated values doesn't explain how this all works, and although there are some bits and pieces in the change tracking section, we're missing a simple, basic section that users would find and understand. EF 8.0 also added the ability to configure the sentinel value, but apart from the what's new section this has not been documented.