docs(graphcache): remove mutative operations

[JoviDeCroock]

It's confusing that we use mutative operations in our updaters when that can alter the base-layer when we i.e. are working with optimistic layers.

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[kitten]

I see the issue here, so basically, a user could accidentally modify a link on a layer by mutating it directly, since cache.resolve returning links is an exception, i.e.: https://github.com/urql-graphql/urql/blob/f06ef3499b69a20896057c0027c5116f4e73c54a/exchanges/graphcache/src/store/store.ts#L161-L162

Does it make sense for us to instead clone links in cache.resolve? It should be the only place that can cause this problem and we already have an appropriate function that can clone links deeply: https://github.com/urql-graphql/urql/blob/f06ef3499b69a20896057c0027c5116f4e73c54a/exchanges/graphcache/src/operations/shared.ts#L226-L249

[JoviDeCroock]

I would personally prefer hammering on the immutable route in the docs either way

[kitten]

I'm fine with changing the docs, yea, let's get this merged as a docs change then, and I'll file the fix above separately, so we don't let this bug become a longer term problem 👍