chore: document change in cache-and-network behavior in v3

[alessbell]

Closes #6760.

Describes a change in behavior of the fetch policy cache-and-network from v2 to v3.
See: #6760 (comment).

Link to the deployed docs page: https://deploy-preview-10714--apollo-client-docs.netlify.app/migrating/apollo-client-3-migration

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[jerelmiller]

Nice! More context is always good. Thanks!!