Updates legacy url aliases dev doc

docs/developer/advanced/legacy-url-aliases.asciidoc

@@ -13,11 +13,20 @@ type are *globally unique across all spaces*.
 {kib} creates a special entity called a **legacy URL alias** for each saved object that requires a new ID. This legacy URL alias allows
 {kib} to preserve any deep link URLs that exist for these objects.
 
+There are two cases where a legacy URL alias will get generated.
+
+1. During migration, when an object's namespace type is being converted from the single-namespace type to a multi-namespace type. If
+the object resides in a non-default space, it gets a new ID and a legacy URL alias is generated.
+
+2. During copy/import of saved objects, when any object requires a new ID **and the compatibilityMode option is enabled** (see
+https://www.elastic.co/guide/en/kibana/current/spaces-api-copy-saved-objects.html#:~:text=the%20createNewCopies%20option.-,compatibilityMode,-(Optional%2C%20boolean)%20Applies [Copy saved objects to space API - compatibilityMode documentation] and https://github.com/elastic/kibana/pull/149021[PR #149021] for more information).
+
 [[legacy-url-aliases-example]]
 === Example
 
-Consider the following scenario:
+Consider the following scenarios:
 
+1. Migration scenario
 You have {kib} 7.16, and you create a new dashboard.The ID of this dashboard is "123". You create a new space called "Bill's space" and
 <<managing-saved-objects-copy-to-space,copy>> your dashboard to the other space. Now you have two different dashboards that can be accessed
 at the following URLs:
@@ -33,6 +42,20 @@ If you use your bookmark to access that dashboard using its old URL, {kib} detec
 ID. If you navigate to `http://localhost:5601/s/bills-space/app/dashboards#/view/123`, you'll see a message indicating that the dashboard
 has a new URL, and you're automatically redirected to `http://localhost:5601/s/bills-space/app/dashboards#/view/456`.
 
+2. Copy scenario/weak links
+You have a data view and two dashboards in the default space, but you would also like to have them in another space. One of the dashboards
+includes a Markdown visualization with a link to the other dashboard - a so-called **weak link**. This is a weak link because the ID of the
+referenced object is not added to the object's references array, and therefore there is no explicit relationship between the objects.
+
+If you were to use the https://www.elastic.co/docs/api/doc/kibana/operation/operation-post-spaces-copy-saved-objects [copy API] to create
+new copies of these assets in another space **without the `compatibilityMode` option set to true**, the Markdown link would be broken. The
+copied objects created in the target space receive a new ID, and the weak link in the Markdown visualization would point to the ID of the
+source object from the originating space.
+
+By setting `compatibilityMode` to true when using the copy API, legacy aliases will be generated for any objects that require a new ID. This
+allows the weak link to the second dashboard to be resolved. Though a dashboard with the ID from the weak link will not be found, a legacy
+alias with this source ID will have been generated, and it will contain a target ID of the new local copy of the dashboard.
+
 [[legacy-url-aliases-handling-errors]]
 === Handling errors