DOCS-16994-dollar-sign-restriction (#11818)

* add note

* add formatting

* change info

* wording

* change db name

Author: shuangela

source/includes/aggregation/agg-dollar-sign.rst

@@ -0,0 +1,12 @@
+.. note:: Dollar Characters in Field Values
+
+    When you use an aggregation pipeline, sanitize any strings that are passed from user
+    input or created dynamically from parsing data. If any field values are literal string
+    values and start with a dollar character, the value must be passed to the
+    :expression:`$literal` aggregation operator. The following example demonstrates using
+    the aggregation pipeline ``$set`` and the ``$literal`` operator to update the document
+    with an ``_id`` of ``1`` to have a ``cost`` field of ``$27``.
+
+    .. code-block:: javascript
+
+       db.inventory.updateOne( { _id: 1 }, [ { $set: { "cost": { $literal: "$27" } } } ] )

source/tutorial/update-documents-with-aggregation-pipeline.txt

@@ -35,6 +35,8 @@ Using the aggregation pipeline allows for a more expressive update
 statement, such as expressing conditional updates based on current
 field values or updating one field using the value of another field(s).
 
+.. include:: /includes/aggregation/agg-dollar-sign.rst
+
 Create an Update Aggregation Pipeline in Atlas
 ----------------------------------------------