docs(documents): add overwriting section

Fix #8178

docs/documents.pug

@@ -142,6 +142,28 @@ block content
 
     Read the [validation](./validation.html) guide for more details.
 
+    ### Overwriting
+
+    There are 2 different ways to overwrite a document (replacing all keys in the
+    document). One way is to use the
+    [`Document#overwrite()` function](/docs/api/document.html#document_Document-overwrite)
+    followed by `save()`.
+
+    ```javascript
+    const doc = await Person.findOne({ _id });
+
+    // Sets `name` and unsets all other properties
+    doc.overwrite({ name: 'Jean-Luc Picard' });
+    await doc.save();
+    ```
+
+    The other way is to use [`Model.replaceOne()`](/docs/api/model.html#model_Model.replaceOne).
+
+    ```javascript
+    // Sets `name` and unsets all other properties
+    await Person.replaceOne({ _id }, { name: 'Jean-Luc Picard' });
+    ```
+
     ### Next Up
 
     Now that we've covered Documents, let's take a look at