az support tickets documentation better to have prerequisites

az support tickets series of commands appears to be depending upon Azure Support REST API.
As REST API has some prerequisites including eligible support plans, CLI documentation also better to have such description.

https://docs.microsoft.com/en-us/rest/api/support/
Prerequisites depend on the API operations you want to call:

To create and update support tickets, and add communication operations, you need:

An Azure subscription ID.
A Professional Direct, Premier, or Unified technical support plan. For more information, see Compare support plans.
Member of the Support Request Contributor role at the subscription scope.
To list support tickets, and to list communication operations, you need:

An Azure subscription ID
Member of the Reader role at the subscription scope.
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#### Document Details

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* ID: cee2f123-dd03-a147-a610-5a3d76570155
* Version Independent ID: ce0c3084-7406-7757-de96-aa003373beda
* Content: [az support tickets](https://docs.microsoft.com/en-us/cli/azure/ext/support/support/tickets?view=azure-cli-latest)
* Content Source: [latest/docs-ref-autogen/ext/support/support/tickets.yml](https://github.com/MicrosoftDocs/azure-docs-cli/blob/live/latest/docs-ref-autogen/ext/support/support/tickets.yml)
* GitHub Login: @rloutlaw
* Microsoft Alias: **routlaw**