DOCS-9744: $sample can only use a random cursor if it is the first st…

source/reference/operator/aggregation/sample.txt

@@ -44,6 +44,16 @@ In order to get N random documents:
   - If using :doc:`/core/mmapv1`, :pipeline:`$sample` uses the ``_id``
     index to randomly select N documents.
 
+:pipeline:`$sample` can use a pseudo-random cursor to find documents
+if:
+
+- It is the first stage in the aggregation pipeline
+- N is less than 5% of the total documents in the collection
+- The collection contains over 100 documents
+
+Otherwise :pipeline:`$sample` performs a random sort on the
+incoming documents and selects the first N documents.
+
 .. warning::
 
    :pipeline:`$sample` may output the same document more than once in