Clarify how viewport for non-square images are handled

Author: mwcraig

src/astro_image_display_api/interface_definition.py

@@ -229,15 +229,16 @@ def set_viewport(
         image_label: str | None = None
     ) -> None:
         """
-        Set the viewport of the image, which defines the center and field of viea.
+        Set the viewport of the image, which defines the center and field of view.
 
         Parameters
         ----------
         center : `astropy.coordinates.SkyCoord` or tuple of float, optional
             The center of the viewport. If not given, the current center is used.
         fov : `astropy.units.Quantity` or float, optional
             The field of view (FOV) of the viewport. If not given, the current FOV
-            is used. If a float is given, it is interpreted a size in pixels.
+            is used. If a float is given, it is interpreted a size in pixels. For images that are
+            not square, the FOV is interpreted as the size of the longer side of the image.
         image_label : str, optional
             The label of the image to set the viewport for. If not given and there is
             only one image loaded, the viewport for that image is set. If there are