docs: clarified azure cloud auto-join requirements

[mattias-fjellstrom]

Description

I set up cloud auto-join on Azure and discovered that the documentation was a bit sparse. For instance, the (required) subscription_id was not listed in the list of properties (but it was listed in the example join string). It was also not immediately clear that the tags had to be on the NICs not on the VMs.

Testing & Reproduction steps

N/A

Links

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[pkazmierczak]

Hey @mattias-fjellstrom, thanks for the contribution!