DOCS-17090 analyzeShardKey defaults (#13649)

Author: kennethdyer

content/manual/upcoming/source/includes/shard-key-characteristics-metrics.rst

@@ -9,11 +9,12 @@ documents sampled from the collection. The calculation requires the
 shard key to have a :ref:`supporting index <supporting-indexes-ref>`. 
 If there is no supporting index, no metrics are returned.
 
-You can configure sampling with the ``sampleRate`` and ``sampleSize`` 
-fields. Both are optional and only one can be specified. If neither is 
-specified, the sample size is set to ``10`` . Configure this value 
-by setting 
-``analyzeShardKeyCharacteristicsDefaultSampleSize``.  
+You can configure sampling with the ``sampleRate`` and
+``sampleSize`` fields. Both are optional, but only one can be
+specified. When both ``sampleRate`` and ``sampleSize`` are
+unspecified, MongoDB uses the value of the
+:parameter:`analyzeShardKeyCharacteristicsDefaultSampleSize`
+parameter, which has a default value of 10 million.
 
 To calculate metrics based on all documents in the collection, 
 set the ``sampleRate`` to ``1``.