Add field.eval info in migration guide

Author: erikvansebille

docs/community/v4-migration.md

@@ -35,6 +35,10 @@ Version 4 of Parcels is unreleased at the moment. The information in this migrat
 - Particlefiles should be created by `ParticleFile(...)` instead of `pset.ParticleFile(...)`
 - The `name` argument in `ParticleFile` has been replaced by `store` and can now be a string, a Path or a zarr store.
 
+## Field
+
+- `Field.eval()` returns an array of floats instead of a single float (related to the vectorization)
+
 ## GridSet
 
 - `GridSet` is now a list, so change `fieldset.gridset.grids[0]` to `fieldset.gridset[0]`.