DOCSP-41055 Betsy comment

Author: ltran-mdb2

source/images/compass/collection-schema.png

@@ -55,6 +55,9 @@ percentage of each data type represented.
      that Compass can provide a graded bar of the field contents, with
      the most-frequently occurring string on the left.
 
+   - The ``grades`` field contains arrays of strings. The analysis
+     shows the minimum, maximum, and average array lengths.
+
    .. figure:: /images/compass/collection-schema.png
       :figwidth: 700px
       :alt: Example of a collection's schema