DOCSP-27256 Add note for collScan workaround (#350)

Author: jordan-smith721

source/usage-examples/count.txt

@@ -23,6 +23,19 @@ than scanning the entire collection. The ``countDocuments()`` method returns
 an **accurate** count of the number of documents and supports specifying
 a filter.
 
+.. tip::
+
+   When using ``countDocuments()`` to return the total number of documents in a
+   collection, you can improve performance by avoiding a collection scan. To do
+   this, use a :manual:`hint </reference/method/cursor.hint>` to take advantage
+   of the built-in index on the ``_id`` field. Use this technique only when
+   calling ``countDocuments()`` with an empty query parameter.
+
+   .. code-block:: java
+
+      CountOptions opts = new CountOptions().hintString("_id_");
+      long numDocuments = collection.countDocuments(new BsonDocument(), opts);
+
 When you call the ``countDocuments()`` method, you can optionally pass a
 **query filter** parameter. You cannot pass any parameters when you call
 ``estimatedDocumentCount()``.