add note about the similarity of $set commands (#6143) (#6210)

* add note about the similarity of $set commands

* fix formatting

Author: MongoCaleb

source/reference/operator/update/set.txt

@@ -149,6 +149,23 @@ After updating, the document has the following values:
       ratings: [ { by: 'Customer007', rating: 4 } ]
    }
 
+.. important::
+
+   The above code uses ``dot notation`` to update the ``make`` field of the 
+   embedded ``details`` document. The code format looks similar to the following 
+   code example, which instead *replaces the entire embedded document*, removing 
+   all other fields in the embedded ``details`` document:
+
+   .. code-block:: javascript
+      :copyable: false
+
+      db.products.updateOne(
+         { _id: 100 },
+         { $set: { details:
+            {make: "Kustom Kidz"}
+            }
+         })
+
 
 Set Elements in Arrays
 ~~~~~~~~~~~~~~~~~~~~~~