Add array examples to reactive variable documentation (#10235)
Add array examples to reactive variable documentation (#10235)

* Add troubleshooting tips to the reactive var documentation.

This PR adds examples on how to avoid some common pitfalls when using reactive vars.  It should hopefully help people get some time back when trying to triage issues with reactive vars mutations not triggering component updates in React applications.

* Updates from PR review:

* Spelling corrections
* Move array example up to Modifying section.

* Remove troubleshooting section, update Modifying section with more concise example.
apollographql/apollo-client,apollographql/apollo-client