Documentation fixes / making availability_set_id conflict with zones

[tombuildsstuff]

Documentation fixes:

• Availability Zones have come out of preview, which fixes Documentation update for availability zones #2151
• Subnet Service Endpoints now has additional values - fixes docs: azurerm_subnet, the service_endpoints option has additional possible values #2144

Also makes availability_set_id conflict with zones in the azurerm_virtual_machine resource

[mbfrahry]

LGTM!

[ghost]

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