Added documentation for Location swap migration (#2440)

Author: mnocon

code_samples/data_migration/examples/swap_location.yaml

@@ -0,0 +1,9 @@
+-
+    type: location
+    mode: swap
+    match1:
+        field: location_remote_id
+        value: f3e90596361e31d496d4026eb624c983
+    match2:
+        field: location_id
+        value: 5

docs/guide/data_migration/exporting_and_importing_data.md

@@ -170,19 +170,19 @@ If you do not provide the `--mode` option, the command asks you to select the mo
 
 The following data migration steps are available:
 
-| `type`               | `create` | `update` | `delete` |
-|----------------------|:--------:|:--------:|:--------:|
-| `content_type`       | ✔ | ✔ |          |
-| `content_type_group` | ✔ | ✔ |          |
-| `content`            | ✔ | ✔ | ✔ |
-| `language`           | ✔ |          |          |
-| `location`           |          | ✔ |          |
-| `object_state`       | ✔ |          |          |
-| `object_state_group` | ✔ |          |          |
-| `role`               | ✔ | ✔ | ✔ |
-| `section`            | ✔ | ✔ |          |
-| `user`               | ✔ | ✔ |          |
-| `user_group`         | ✔ |          | ✔ |
+| `type`               | `create` | `update` | `delete` | `swap`   |
+|----------------------|:--------:|:--------:|:--------:|:--------:|
+| `content_type`       | ✔ | ✔ |          |          |
+| `content_type_group` | ✔ | ✔ |          |          |
+| `content`            | ✔ | ✔ | ✔ |          |
+| `language`           | ✔ |          |          |          |
+| `location`           |          | ✔ |          | ✔ |
+| `object_state`       | ✔ |          |          |          |
+| `object_state_group` | ✔ |          |          |          |
+| `role`               | ✔ | ✔ | ✔ |          |
+| `section`            | ✔ | ✔ |          |          |
+| `user`               | ✔ | ✔ |          |          |
+| `user_group`         | ✔ |          | ✔ |          |
 
 ## siteaccess
 
@@ -364,6 +364,16 @@ The required metadata keys are: `identifier`, `mainTranslation`, `contentTypeGro
 [[= include_file('code_samples/data_migration/examples/create_blog_post_ct.yaml') =]]
 ```
 
+### Locations
+
+The following example shows how to swap content items assigned to given locations.
+
+``` yaml
+[[= include_file('code_samples/data_migration/examples/swap_location.yaml') =]]
+```
+
+
+
 ### Images
 
 The following example shows how to migrate an `example-image.png` located in