Fix/improve documentation regarding infrastructure config

[vpnachev]

How to categorize this issue?

/area dev-productivity documentation ops-productivity usability
/kind bug
/platform azure

What happened:
Documentation and code around infrastructure config needs various fixes/improvements

---

gardener-extension-provider-azure/docs/usage/usage.md

Lines 151 to 156 in 25b3283

	vnet: # specify either 'name' and 'resourceGroup' or 'cidr'
	# name: my-vnet
	# resourceGroup: my-vnet-resource-group
	cidr: 10.250.0.0/16
	# ddosProtectionPlanID: /subscriptions/test/resourceGroups/test/providers/Microsoft.Network/ddosProtectionPlans/test-ddos-protection-plan
	workers: 10.250.0.0/19

states that either cidr or name and resourceGroup must be set, however it turns out none of them can be uset, in that case networks.workersis re-used ascidr`, this needs to be documented.

Should be fixed also at

gardener-extension-provider-azure/docs/usage/usage.md

Lines 193 to 195 in 25b3283

	* If `networks.vnet.cidr` is given then you have to specify the VNet CIDR of a new VNet that will be created during shoot creation.
	You can freely choose a private CIDR range.
	* Either `networks.vnet.name` and `neworks.vnet.resourceGroup` or `networks.vnet.cidr` must be present, but not both at the same time.

---

gardener-extension-provider-azure/pkg/apis/azure/validation/infrastructure.go

Lines 190 to 193 in 25b3283

	// Validate that just vnet name or vnet resource group is specified.
	if (networkConfig.VNet.Name != nil && vnetConfig.ResourceGroup == nil) || (vnetConfig.Name == nil && vnetConfig.ResourceGroup != nil) {
	return append(allErrs, field.Invalid(vNetPath, vnetConfig, "a vnet cidr or vnet name and resource group need to be specified"))
	}

Comment and the error message does not correspond to the actual check, both of them must be fixed especially the error message because it is user facing.

---

What you expected to happen:
Documentation, end-user documentation in in-code comments, to reflect what the code is actually doing.

How to reproduce it (as minimally and precisely as possible):

Anything else we need to know?:

Environment:

• Gardener version (if relevant):
• Extension version:
• Kubernetes version (use kubectl version):
• Cloud provider or hardware configuration:
• Others: