chore: add migration guide for breaking change to get_ancestors

Author: bhearsum

docs/reference/migrations.rst

@@ -3,6 +3,28 @@ Migration Guide
 
 This page can help when migrating Taskgraph across major versions.
 
+12.x -> 13.x
+------------
+
+* Invert any usage of the dict keys and values returned by `get_ancestors`:
+
+  For example, if you were using:
+
+  .. code-block:: python
+
+    for label, taskid in get_ancestors(...):
+      ...
+
+  Change it to:
+
+  .. code-block:: python
+
+    for taskid, label in get_ancestors(...):
+      ...
+
+  Note that due to this change `get_ancestors` may return multiple tasks with
+  the same label now, which your code may need to deal with.
+
 11.x -> 12.x
 ------------