Clarify defaultValue usage (#868)

Fixes #867

content/docs/context.md

@@ -48,7 +48,7 @@ const {Provider, Consumer} = React.createContext(defaultValue);
 
 Creates a `{ Provider, Consumer }` pair. When React renders a context `Consumer`, it will read the current context value from the closest matching `Provider` above it in the tree.
 
-The `defaultValue` argument is used when you render a Consumer without a matching Provider above it in the tree. This can be helpful for testing components in isolation without wrapping them.
+The `defaultValue` argument is **only** used by a Consumer when it does not have a matching Provider above it in the tree. This can be helpful for testing components in isolation without wrapping them. Note: passing `undefined` as a Provider value does not cause Consumers to use `defaultValue`.
 
 ### `Provider`