Add a note about upstream bug

Author: seisman

pygmt/src/grdview.py

@@ -115,8 +115,11 @@ def grdview(  # noqa: PLR0913
         **m** or **sm** for meshlines to be drawn.
     plane
         Draw a plane at the specified z-level. If ``True``, defaults to the minimum
-        value in the grid. Use ``facade_pen`` and ``facade_fill`` to control the
-        appearance of the plane.
+        value in the grid. However, if ``region`` was used to set *zmin/zmax* then
+        *zmin* is used if it is less than the grid minimum value. Use ``facade_pen`` and
+        ``facade_fill`` to control the appearance of the plane.
+        **Note**: For GMT<=6.6.0, *zmin* set in ``region`` has no effect due to a GMT
+        bug.
     facade_fill
         Fill for the frontal facade between the plane specified by ``plane`` and the
         data perimeter.