Add x-ms-internal extension (#4746)

docs/extensions/readme.md

@@ -26,6 +26,7 @@ The following documents describes AutoRest specific vendor extensions for [OpenA
 - [x-ms-long-running-operation](#x-ms-long-running-operation) - indicates that the operation implemented Long Running Operation pattern as defined by the [Resource Manager API](https://msdn.microsoft.com/en-us/library/azure/dn790568.aspx).
 - [x-nullable](#x-nullable) - when `true`, specifies that `null` is a valid value for the associated schema
 - [x-ms-header-collection-prefix](#x-ms-header-collection-prefix) - Handle collections of arbitrary headers by distinguishing them with a specified prefix.
+- [x-ms-internal](#x-ms-internal) - allow specifc operations not be exposed to users in generated client
 
 ### Microsoft Azure Extensions (available in most generators only when using `--azure-arm`)
 
@@ -1477,6 +1478,27 @@ headers:
 
 What is returned to users is just `key: value`.
 
+## x-ms-internal
+
+When an operation contains this extension, it will be generated but not be exposed to user in generated client. This extension is always used in situation when an operation needs to be wrapped by manual customization code.
+
+**Schema**:
+`true|false`
+
+**Example**:
+
+```json5
+"paths": {
+  "/test": {
+    "put": {
+      "operationId": "test_put",
+      "x-ms-internal": true,
+      "description": "This operation will not be exposed to user in client"
+    }
+  }
+}
+```
+
 # Metadata Extensions
 
 ## x-ms-secret