Add a migration guide.

Author: andriyDev

release-content/migration-guides/extract_fn_is_mut.md

@@ -0,0 +1,10 @@
+---
+title: `take_extract` now returns `dyn FnMut` instead of `dyn Fn`.
+pull_requests: [19926]
+---
+
+Previously, `set_extract` accepted any `Fn`. Now we accept any `FnMut`. For callers of
+`set_extract`, there is no difference since `Fn: FnMut`.
+
+However, callers of `take_extract` will now be returned
+`Option<Box<dyn FnMut(&mut World, &mut World) + Send>>` instead.