Document optimisticResponse handling of GraphQL errors in optimistic UI (#13000)

* Document handling of GraphQL errors in optimistic UI

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

* Use active voice

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Co-authored-by: Jerel Miller <[email protected]>

Author: sbatson5

docs/source/performance/optimistic-ui.mdx

@@ -73,6 +73,8 @@ As this example shows, the value of `optimisticResponse` is an object that match
 
 5. Apollo Client notifies all affected queries again. The associated components re-render, but if the server's response matches our `optimisticResponse`, this is invisible to the user.
 
+6. If the mutation returns a GraphQL error, Apollo Client discards the optimistic version of the object and rolls back to the previous state.
+
 ## Bailing out of an optimistic update
 
 In some cases you may want to skip an optimistic update. For example, you may want to perform an optimistic update _only_ when certain variables are passed to the mutation. To skip an update, pass a function to the `optimisticResponse` option and return the `IGNORE` sentinel object available on the second argument to bail out of the optimistic update.