(DOCS-14745): Added note about the document count. (#355)

* (DOCS-14745): Added note about the document count.

* (DOCS-14745): Added more details per Moe's question.

* (DOCSP-14745): Incorporated info from Sergey

Author: corryroot

source/collections.txt

@@ -35,6 +35,19 @@ information for each collection in the selected database:
 
 - Collection name
 - Number of :doc:`documents </documents>` in the collection
+ 
+  .. note::
+
+    |compass-short| bases the document count that appears on the 
+    :guilabel:`Collections` screen on cached metadata using 
+    :manual:`collStats </reference/command/collStats/>`. This count 
+    might differ from the actual document count in the collection. For 
+    example, an :manual:`unexpected shutdown 
+    </reference/command/collStats/#behavior>` can throw off the count. 
+    Use the :manual:`db.collection.countDocuments() 
+    </reference/method/db.collection.countDocuments/>` method for the 
+    most accurate document count.
+
 - Average size of documents in the collection
 - Total size of all documents in the collection
 - Number of :doc:`indexes </indexes>` on the collection