Update core/nwb.base.yaml

Co-authored-by: Steph Prince <[email protected]>

Author: rly

core/nwb.base.yaml

@@ -65,7 +65,8 @@ datasets:
 - neurodata_type_def: ExternalImage
   neurodata_type_inc: BaseImage
   doc: A type for referencing an external image file. The single file path or URL to the external image file should
-    be stored in the dataset.
+    be stored in the dataset. This type should NOT be used if the image is stored 
+     in another NWB file and that file is linked to this file.
   dtype: text
   # shape: scalar  # this will be supported in the NWB schema language 2.0