DOCSP-1173 Updated according to reviewer's suggestions.

Author: Irene Smith

source/reference/operator/aggregation/bucketAuto.txt

@@ -50,8 +50,7 @@ Definition
 Behavior
 --------
 
-The number of buckets in the output may be less than the specified
-number of buckets, for example:
+There may be less than the specified number of buckets if:
 
 - The number of input documents is less than the specified number of
   buckets.
@@ -69,6 +68,11 @@ If the ``groupBy`` expression refers to an array or document, the
 values are arranged using the same ordering as in :pipeline:`$sort`
 before determining the bucket boundaries.
 
+The even distribution of documents across buckets depends on the 
+cardinality, or the number of unique values, of the ``groupBy`` field. If 
+the cardinality is not high enough, the $bucketAuto stage may not evenly 
+distribute the results across buckets.
+
 Granularity
 ~~~~~~~~~~~