(DOCS-14763): Clarify sample agg behavior warning

Author: jeff-allen-mongo

source/reference/operator/aggregation/sample.txt

@@ -44,10 +44,18 @@ collection scan followed by a random sort to select N documents. In
 this case, the :pipeline:`$sample` stage is subject to the
 :ref:`sort memory restrictions <sort-memory-limit>`.
 
-.. warning::
+MMAPV1 May Return Duplicate Documents
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-   :pipeline:`$sample` may output the same document more than once in
-   its result set.
+If you are using the:
+
+- MMAPV1 storage engine, :pipeline:`$sample` may return the same
+  document more than once in the result set.
+
+- :ref:`WiredTiger <storage-wiredtiger>` or
+  :ref:`in-memory <storage-inmemory>` storage engine,
+  :pipeline:`$sample` does not return duplicate documents. WiredTiger
+  is the default storage engine as of MongoDB 3.2.
 
 Example
 --------