ioredis migration docs suggestions

[PavelPashov]

Below are some findings and suggestions for improving the ioredis migration docs

ioredis migration suggestions

Initial Connection

The below await in the node-redis section is redundant

const client = await createClient();

Asynchronous command result handling

node-redis supports callbacks via Legacy Mode

SETNX command

node-redis has a SETNX command

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[andy-stark-redis]

A few small suggestions, but basically LGTM.