Document waitUntil function present in the server context

[EmrysMyrddin]

whatwg-node provides a waitUntil function in the context. This should be used to register promises that are not directly awaited in the code.

We should:

• Surface it in the YogaInitialContext type
• Add documentation about its usage and recommendations when needed
• Use it in useResponseCache to keep track of set and invalidate calls