rpc: Document what is actually going on with stream optimism.

Nick Mathewsonrequested to merge
nickm/arti:rpc_document_internal_optimism into main

In brief: we do correct implement correct optimistic vs nonoptimistic behavior for RPC streams. Only our documentation was wrong. Subsequent commits will fix our documentation more.

Closes #1583 (closed).

Edited by Nick Mathewson

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