docs(queries): clarify when to use queries versus aggregations

Fix #8494

Author: vkarpov15

docs/queries.pug

@@ -51,6 +51,7 @@ block content
       <li><a href="#connection-string-options">Connection String Options</a></li>
       <li><a href="#refs">References to other documents</a></li>
       <li><a href="#streaming">Streaming</a></li>
+      <li><a href="#versus-aggregation">Versus Aggregation</a></li>
     </ul>
 
     ### Executing
@@ -176,6 +177,48 @@ block content
     });
     ```
 
+    <h3 id="versus-aggregation"><a href="#versus-aggregation">Versus Aggregation</a></h3>
+
+    [Aggregation](https://mongoosejs.com/docs/api.html#aggregate_Aggregate) can
+    do many of the same things that queries can. For example, below is
+    how you can use `aggregate()` to find docs where `name.last = 'Ghost'`:
+
+    ```javascript
+    const docs = await Person.aggregate([{ $match: { 'name.last': 'Ghost' } }]);
+    ```
+
+    However, just because you can use `aggregate()` doesn't mean you should.
+    In general, you should use queries where possible, and only use `aggregate()`
+    when you absolutely need to.
+
+    Unlike query results, Mongoose does **not** [`hydrate()`](/docs/api/model.html#model_Model.hydrate)
+    aggregation results. Aggregation results are always POJOs, not Mongoose
+    documents.
+
+    ```javascript
+    const docs = await Person.aggregate([{ $match: { 'name.last': 'Ghost' } }]);
+
+    docs[0] instanceof mongoose.Document; // false
+    ```
+
+    Also, unlike query filters, Mongoose also doesn't
+    [cast](/docs/tutorials/query_casting.html) aggregation pipelines. That means
+    you're responsible for ensuring the values you pass in to an aggregation
+    pipeline have the correct type.
+
+    ```javascript
+    const doc = await Person.findOne();
+
+    const idString = doc._id.toString();
+
+    // Finds the `Person`, because Mongoose casts `idString` to an ObjectId
+    const queryRes = await Person.findOne({ _id: idString });
+
+    // Does **not** find the `Person`, because Mongoose doesn't cast aggregation
+    // pipelines.
+    const aggRes = await Person.aggregate([{ $match: { _id: idString } }])
+    ```
+
     <h3 id="next"><a href="#next">Next Up</a></h3>
 
     Now that we've covered `Queries`, let's take a look at [Validation](/docs/validation.html).