Fix import and docstring

Author: VeckoTheGecko

src/parcels/_core/fieldset.py

@@ -11,6 +11,7 @@
 import xgcm
 
 from parcels._core.field import Field, VectorField
+from parcels._core.utils import sgrid
 from parcels._core.utils.string import _assert_str_and_python_varname
 from parcels._core.utils.time import get_datetime_type_calendar
 from parcels._core.utils.time import is_compatible as datetime_is_compatible
@@ -302,6 +303,8 @@ def from_sgrid_conventions(
 
         This is the primary ingestion method in Parcels for structured grid datasets.
 
+        Assumes that U, V, (and optionally W) variables are named 'U', 'V', and 'W' in the dataset.
+
         Parameters
         ----------
         ds : xarray.Dataset