Add documentation for CloudConfiguration (#987)

* rm trailing whitespaces

* add doc for CloudConfiguration usage

docs/usage/usage.md

@@ -187,15 +187,15 @@ infrastructureConfig:
     zones:
       - name: 3
         cidr: 10.250.0.0/19 # note the preservation of the 'workers' CIDR
-# optionally add other zones 
-    # - name: 2  
+# optionally add other zones
+    # - name: 2
     #   cidr: 10.250.32.0/19
     #   natGateway:
     #     enabled: true
   zoned: true
 ```
 
-Another more advanced example with user-provided public IP addresses for the NAT Gateway and how it can be migrated: 
+Another more advanced example with user-provided public IP addresses for the NAT Gateway and how it can be migrated:
 
 ```yaml
 infrastructureConfig:
@@ -240,8 +240,8 @@ infrastructureConfig:
             - name: pip2
               resourceGroup: group
               zone: 1
-# optionally add other zones 
-#     - name: 2  
+# optionally add other zones
+#     - name: 2
 #       cidr: 10.250.32.0/19
 #       natGateway:
 #         enabled: true
@@ -314,7 +314,7 @@ The `cloudControllerManager.featureGates` contains a map of explicitly enabled o
 For production usage it's not recommend to use this field at all as you can enable alpha features or disable beta/stable features, potentially impacting the cluster stability.
 If you don't want to configure anything for the `cloudControllerManager` simply omit the key in the YAML specification.
 
-`storage` contains options for storage-related control plane component. 
+`storage` contains options for storage-related control plane component.
 `storage.managedDefaultStorageClass` is enabled by default and will deploy a `storageClass` and mark it as a default (via the `storageclass.kubernetes.io/is-default-class` annotation)
 `storage.managedDefaultVolumeSnapshotClass` is enabled by default and will deploy a `volumeSnapshotClass` and mark it as a default (via the `snapshot.storage.kubernetes.io/is-default-classs` annotation)
 In case you want to manage your own default `storageClass` or `volumeSnapshotClass` you need to disable the respective options above, otherwise reconciliation of the controlplane may fail.
@@ -366,7 +366,7 @@ dataVolumes:
       # urn: sap:gardenlinux:greatest:1443.10.0
 ```
 
-The `.nodeTemplate` is used to specify resource information of the machine during runtime. This then helps in Scale-from-Zero. 
+The `.nodeTemplate` is used to specify resource information of the machine during runtime. This then helps in Scale-from-Zero.
 Some points to note for this field:
 - Currently only cpu, gpu and memory are configurable.
 - a change in the value lead to a rolling update of the machine in the worker pool
@@ -597,6 +597,25 @@ The prerequisites are that the cluster must be zoned, and the used machine type
 `Availability Set` based shoot clusters will not be enabled for accelerated networking even if the machine type and operating system support it, this is necessary because all machines from the availability set must be scheduled on special hardware, more daitls can be found [here](https://github.com/MicrosoftDocs/azure-docs/issues/10536).
 Supported machine types are listed in the CloudProfile in `.spec.providerConfig.machineTypes[].acceleratedNetworking` and the supported operating system image versions are defined in `.spec.providerConfig.machineImages[].versions[].acceleratedNetworking`.
 
+### Support for other Azure instances
+
+The provider extension can be configured to connect to Azure instances other than the public one by providing additional configuration in the CloudProfile:
+```yaml
+spec:
+  …
+  providerConfig:
+    apiVersion: azure.provider.extensions.gardener.cloud/v1alpha1
+    kind: CloudProfileConfig
+    cloudConfiguration:
+      name: AzurePublic # AzurePublic | AzureGovernment | AzureChina
+    machineTypes:
+      …
+    …
+  …
+```
+If no configuration is specified the extension will default to the public instance.
+Azure instances other than `AzurePublic`, `AzureGovernment`, or `AzureChina` are not supported at this time.
+
 ### Preview: Shoot clusters with VMSS Flexible Orchestration (VMSS Flex/VMO)
 
 The machines of an Azure cluster can be created while being attached to an [Azure Virtual Machine ScaleSet with flexible orchestraion](https://docs.microsoft.com/en-us/azure/virtual-machine-scale-sets/virtual-machine-scale-sets-orchestration-modes#scale-sets-with-flexible-orchestration).