add reFetchObservableQueries to ApolloClient docs

[michaelknoch]

I wondered why the ApolloClient.reFetchObservableQueries is not part of the documentation because its public api. Would love to see this merged.

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests

[apollo-cla]

@michaelknoch: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Meteor Contributor Agreement here: https://contribute.meteor.com/

[danilobuerger]

Thanks @michaelknoch