docs(middleware): add note about accessing the document being updated in pre('findOneAndUpdate')

Fix #8218

Author: vkarpov15

docs/middleware.pug

@@ -360,11 +360,22 @@ block content
     **query** object rather than the document being updated.
 
     For instance, if you wanted to add an `updatedAt` timestamp to every
-    `update()` call, you would use the following pre hook.
+    `updateOne()` call, you would use the following pre hook.
 
     ```javascript
-    schema.pre('update', function() {
-      this.update({},{ $set: { updatedAt: new Date() } });
+    schema.pre('updateOne', function() {
+      this.set({ updatedAt: new Date() });
+    });
+    ```
+
+    You **cannot** access the document being updated in `pre('updateOne')` or
+    `pre('findOneAndUpdate')` middleware. If you need to access the document
+    that will be updated, you need to execute an explicit query for the document.
+
+    ```javascript
+    schema.pre('findOneAndUpdate', async function() {
+      const docToUpdate = await this.model.findOne(this.getQuery());
+      console.log(docToUpdate); // The document that `findOneAndUpdate()` will modify
     });
     ```