azuredns: update docs

[jlaundry]

• Clarify that specifying the target DNS zone's Tenant ID, Subscription ID, and Resource Group Name is required to use this provider with Managed Identities.
• Provide an example of how to set permissions on a DNS Zone so that a Managed Identity can request a certificate.
• Provide a hint for people trying to use Azure Arc's Managed Identity, that they may need to set special environment variables.

Pinging @pchanvallon for a review (and thanks for your efforts building this provider!)

[ldez]

Hello,

this file is generated:

lego/docs/content/dns/zz_gen_azuredns.md

Lines 12 to 14 in 07c4dae

	<!-- THIS DOCUMENTATION IS AUTO-GENERATED. PLEASE DO NOT EDIT. -->
	<!-- providers/dns/azuredns/azuredns.toml -->
	<!-- THIS DOCUMENTATION IS AUTO-GENERATED. PLEASE DO NOT EDIT. -->

Could you update the source file: https://github.com/go-acme/lego/blob/master/providers/dns/azuredns/azuredns.toml

[jlaundry]

🤦🏻‍♂️ very sorry, updated now

[pchanvallon]

LGTM ✅