Update doc

spec/ndx-extracellular-channels.extensions.yaml

@@ -155,15 +155,13 @@ groups:
     dtype: float
     doc: Dorsoventral (DV) stereotactic coordinate of where the probe was inserted,
       in millimeters. + is up. Coordinate is relative to the zero-point described
-      in `position_reference`.
+      in `position_reference`. The zero-point is typically the surface of the brain,
+      so this value is typically 0.
     required: false
   - name: depth_in_mm
     dtype: float
-    doc: Depth that the probe was driven along `insertion_angle` starting from `insertion_position_ap_in_mm`
-      and `insertion_position_ml_in_mm`, in millimeters. This is an alternate method
-      of providing the dorsal-ventral coordinate of the probe insertion site. If both
-      `insertion_position_dv_in_mm` and `depth_in_mm` are provided, the values should
-      be consistent.
+    doc: Depth that the probe was driven along `insertion_angle` starting from `insertion_position_{X}_in_mm`,
+      in millimeters.
     required: false
   - name: position_reference
     dtype: text