Add missing assumeImmutableResults documentation

[henryqdineen]

A colleague has been profiling one of our apps and has found that setting assumeImmutableResults: true has a positive impact to performance. We noticed that this option is not in the documentation so I opened this PR. Is this intentionally undocumented?

I also wonder, if using InMemoryCache is there any reason to not set assumeImmutableResults: true? Let me know if there's something obvious that I'm missing. Thanks!

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

[benjamn]

Oh wow, I totally assumed this option defaulted to true after PR #5153, but no! Thanks for catching this @henryqdineen!

[henryqdineen]

Thanks Ben! Credit to @tsbehlman who brought it to my attention.

@benjamn So am I correct that we can safely set this to true in all of our apps? Is there anything we should watch out for?

[benjamn]

@henryqdineen Yes, as long as you're using a cache like InMemoryCache that freezes its results, or you're careful about never modifying cache result objects in your application, it should be safe to pass assumeImmutableResults: true to the ApolloClient constructor.