add doc

Signed-off-by: Nester.zhou <ester.zhou@huawei.com>

en/application-dev/reference/apis/js-apis-environment.md

+# Environment
+
+> ![icon-note.gif](public_sys-resources/icon-note.gif) **Note:**
+> The initial APIs of this module are supported since API version 8. Newly added APIs will be marked with a superscript to indicate their earliest API version.
+
+## Modules to Import
+
+```js
+import environment from '@ohos.environment';
+```
+
+## System Capabilities
+
+SystemCapability.FileManagement.File.Environment
+
+## environment.getStorageDataDir
+
+getStorageDataDir():Promise<string>
+
+Obtains the root directory of the storage. This method uses a promise to return the result.
+
+- Return value
+
+  | Type| Description|
+  | --------------------- | ---------------- |
+  | Promise<string> | Promise returned with the root directory of the storage.|
+
+- Example
+
+  ```js
+  environment.getStorageDataDir().then(function(path){
+      console.info("getStorageDataDir successfully:"+ path);
+  }).catch(function(error){
+      console.info("getStorageDataDir failed with error:"+ error);
+  });
+  ```
+
+## environment.getStorageDataDir
+
+getStorageDataDir(callback:AsyncCallback<string>):void
+
+Obtains the root directory of the storage. This method uses an asynchronous callback to return the result.
+
+- Parameters
+
+  | Name| Type| Mandatory| Description|
+  | -------- | --------------------------- | ---- | -------------------------------- |
+  | callback | AsyncCallback<string> | Yes| Asynchronous callback used to return the root directory of the storage.|
+
+- Example
+
+  ```js
+  environment.getStorageDataDir(function(error, path){
+      // do something
+  });
+  ```
+
+## environment.getUserDataDir
+
+getUserDataDir():Promise<string>
+
+Obtains the root directory of public files. This method uses a promise to return the result.
+
+- Return value
+
+  | Type| Description|
+  | --------------------- | ------------------ |
+  | Promise<string> | Promise returned with the root directory of public files.|
+
+- Example
+
+  ```js
+  environment.getUserDataDir().then(function(path){
+      console.info("getUserDataDir successfully:"+ path);
+  }).catch(function(error){
+      console.info("getUserDataDir failed with error:"+ error);
+  });
+  ```
+
+## environment.getUserDataDir
+
+getUserDataDir(callback:AsyncCallback<string>): void
+
+Obtains the root directory of public files. This method uses an asynchronous callback to return the result.
+
+- Parameters
+
+  | Name| Type| Mandatory| Description|
+  | -------- | --------------------------- | ---- | -------------------------------- |
+  | callback | AsyncCallback<string> | Yes| Asynchronous callback used to return the root directory of public files.|
+  
+- Example
+
+  ```js
+  environment.getUserDataDir(function(error, path){
+      // do something
+  });
+  ```