az iot central app device-twin : Update documentation for Aug update

CLI improvements
To expand the capabilities of the CLI, we've re-organized the commands in the azure-iot extension. New commands include az iot central user, az iot central api-token, and az iot central device compute-device-key to generate a device SAS key for provisioning.

Two commands have moved to a new area:

**az iot central app device-twin has moved to az iot central device twin.
az iot central app monitor-events has moved to az iot central diagnostics monitor-events.**

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#### Document Details

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* ID: 8ca03657-212b-3c2e-aaf6-d7a1d3d91db6
* Version Independent ID: e11da47a-6a74-e46e-baeb-5b347415f9d9
* Content: [az iot central app device-twin](https://docs.microsoft.com/en-us/cli/azure/ext/azure-iot/iot/central/app/device-twin?view=azure-cli-latest)
* Content Source: [latest/docs-ref-autogen/ext/azure-iot/iot/central/app/device-twin.yml](https://github.com/MicrosoftDocs/azure-docs-cli/blob/master/latest/docs-ref-autogen/ext/azure-iot/iot/central/app/device-twin.yml)
* GitHub Login: @rloutlaw
* Microsoft Alias: **routlaw**