Add Documentation for PUT Requests to Feature Flag User Lists API

doc/api/feature_flag_user_lists.md

-# Feature Flag User Lists API **(PREMIUM)**
+# Feature flag user lists API **(PREMIUM)**
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/205409) in [GitLab Premium](https://about.gitlab.com/pricing/) 12.10.
 
@@ -63,7 +63,7 @@ POST /projects/:id/feature_flags_user_lists
 | ------------------- | ---------------- | ---------- | ---------------------------------------------------------------------------------------|
 | `id`                | integer/string   | yes        | The ID or [URL-encoded path of the project](README.md#namespaced-path-encoding).       |
 | `name`              | string           | yes        | The name of the feature flag. |
-| `user_xids`         | string           | yes        | A comma separated list of user ids. |
+| `user_xids`         | string           | yes        | A comma separated list of user IDs. |
 
 ```shell
 curl https://gitlab.example.com/api/v4/projects/1/feature_flags_user_lists \
@@ -122,6 +122,47 @@ Example response:
 }
 ```
 
+## Update a feature flag user list
+
+Updates a feature flag user list.
+
+```plaintext
+PUT /projects/:id/feature_flags_user_lists/:iid
+```
+
+| Attribute           | Type             | Required   | Description                                                                            |
+| ------------------- | ---------------- | ---------- | ---------------------------------------------------------------------------------------|
+| `id`                | integer/string   | yes        | The ID or [URL-encoded path of the project](README.md#namespaced-path-encoding).       |
+| `iid`               | integer/string   | yes        | The internal ID of the project's feature flag user list.                               |
+| `name`              | string           | no         | The name of the feature flag.                                                          |
+| `user_xids`         | string           | no         | A comma separated list of user IDs.                                                    |
+
+```shell
+curl https://gitlab.example.com/api/v4/projects/1/feature_flags_user_lists/1 \
+     --header "PRIVATE-TOKEN: <your_access_token>" \
+     --header "Content-type: application/json" \
+     --request PUT \
+     --data @- << EOF
+{
+    "user_xids": "user2,user3,user4"
+}
+EOF
+```
+
+Example response:
+
+```json
+{
+   "name": "my_user_list",
+   "user_xids": "user2,user3,user4",
+   "id": 1,
+   "iid": 1,
+   "project_id": 1,
+   "created_at": "2020-02-04T08:32:27.288Z",
+   "updated_at": "2020-02-05T09:33:17.179Z"
+}
+```
+
 ## Delete feature flag user list
 
 Deletes a feature flag user list.