[Docs]: add explanations of the actual behavior of getSlots to docs

[AkiraVoid]

Discussed in https://github.com/microsoft/fluentui/discussions/24290

^{Originally posted by AkiraVoid August 9, 2022} In v9, the default value of a slot should obviously be undefined, because:

• Users can pass something to slots to customize them.
• Users can pass a null to defend them from being rendered.
• Developers can let them have default rendering.

But I actually got a default value null, is that correct?

People use this approach to build their own components, not only getSlots but the whole mechanism should be explained more clearly.