Clarify use of extensions in context for prefetch.

[lmckenzi]

At present, the specification says Only root-level fields with a primitive value within the context object are eligible to be used as prefetch tokens. This would seem to preclude the use of extension elements in context from being used in prefetch. That's a pretty fundamental limit on extensibility.

Suggest changing the text to Only root-level fields and context.extension root-level fields with a primitive value...