doc: mention iot hub query in iot hub device-twin list help

[Delapouite]

Hi

While trying to query devices from the IoTHub registry my first attempt was to look for a az iot hub device-twin query command which does not exist.

Then, I stumbled upon az iot hub device-twin list which does a select * from devices by default. But looking at the auto-completion of this command I was a bit misled : it does indeed offer a --query flag, but it has nothing to do with the fact of querying devices. As discovered later in the --help output, it's a global flag also used by a lot of commands to filter the JSON output with JMESPath.

In the end, the command I was looking for is az iot hub query which was not very obvious as there's no mention of "devices" in its name.

So my hope with this PR is that it will avoid other users to waste time and more easily find the command they are looking for.

Thanks.

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