Document optimisticResponse handling of GraphQL errors in optimistic UI

[sbatson5]

What's in this PR?

Add explanation for handling GraphQL errors in optimistic updates. It wasn't obvious what happens when a mutation fails and error is returned from the server. Adding one extra item in the lifecycle explanation makes it much clearer what happens

[apollo-cla]

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Latest commit: 601039c

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[jerelmiller]

Small suggestion to use active voice, but otherwise looks great! Thanks for the contribution!