docs: add note about passing context as a function (#757)
docs: add note about passing context as a function (#757)

* docs: add note about passing context as a function

We realized today (by mistake) that the value of `context` in `GraphQLOptions` can be a function. Adding a note to the docs so it doesn't surprise anyone else.

* docs: update context as a function docs

- fix description per @n1ru4l's feedback
- add a code example of instantiating a new class in the context for each request

apollostack/apollo-server