chore(docs): Clarify usage of getDataFromTree

[DanForys]

Hello Apollo-client contributors!

This PR clarifies the docs around getDataFromTree to make clear that renderToStringWithData should be used instead, when rendering the tree directly to a string.

In my case, I spent half a day trying to figure out why React was giving me hydration mismatch warnings once I'd moved over to Apollo SSR. It was only after finding https://lihautan.com/hydrating-text-content/ I figured out the right thing to search for within the Apollo API and thought the main SSR docs would benefit from a small clarification.

Thanks!

[apollo-cla]

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

[benjamn]

Good catch—thanks @DanForys!