DOCS-14442 $in operator performance limitation (#258)

* DOCS-14442  operator limitation

* DOCS-14442 Fixes from internal review

* DOCS-14442 Refactor per Nic Feedback

* DOCS-14442 Refactor per Nic Feedback

* DOCS-14442 Refactor per Nic Feedback

* DOCS-14442 Refactor per Nic Feedback

Author: kennethdyer

source/reference/operator/query/in.txt

@@ -27,6 +27,16 @@ $in
    at least one element that matches a value in the specified array
    (for example, ``<value1>``, ``<value2>``, and so on).
 
+   The :query:`$in` operator compares each parameter to each document
+   in the collection, which can lead to performance issues.
+   To improve performance:
+
+   * It is recommended that you limit the number of parameters passed to the
+     :query:`$in` operator to tens of values.  Using hundreds of parameters
+     or more can negatively impact query performance.
+
+   * Create an index on the ``field`` you want to query.
+
    .. note::
 
       This document describes the :query:`$in` query operator.