docs(schematypes): add some examples of getters and warning about using map() getters with array paths

Fix #6637

Author: vkarpov15

docs/schematypes.jade

@@ -10,7 +10,7 @@ block content
     SchemaTypes handle definition of path
     [defaults](./api.html#schematype_SchemaType-default),
     [validation](./api.html#schematype_SchemaType-validate),
-    [getters](./api.html#schematype_SchemaType-get),
+    [getters](#getters),
     [setters](./api.html#schematype_SchemaType-set),
     [field selection defaults](./api.html#schematype_SchemaType-select) for
     [queries](./api.html#query-js),
@@ -375,6 +375,68 @@ block content
     Keys in a BSON object are ordered, so this means the [insertion order](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map#Description)
     property of maps is maintained.
 
+    <h3 id="getters"><a href="#getters">Getters</a></h3>
+
+    Getters are like virtuals for paths defined in your schema. For example,
+    let's say you wanted to store user profile pictures as relative paths and
+    then add the hostname in your application. Below is how you would structure
+    your `userSchema`:
+
+    ```javascript
+    const root = 'https://s3.amazonaws.com/mybucket';
+
+    const userSchema = new Schema({
+      name: String,
+      picture: {
+        type: String,
+        get: v => `${root}${v}`
+      }
+    });
+
+    const User = mongoose.model('User', userSchema);
+
+    const doc = new User({ name: 'Val', picture: '/123.png' });
+    doc.picture; // 'https://s3.amazonaws.com/mybucket/123.png'
+    doc.toObject({ getters: false }).picture; // '123.png'
+    ```
+
+    Generally, you only use getters on primitive paths as opposed to arrays
+    or subdocuments. Because getters override what accessing a Mongoose path returns,
+    declaring a getter on an object may remove Mongoose change tracking for
+    that path.
+
+    ```javascript
+    const schema = new Schema({
+      arr: [{ url: String }]
+    });
+
+    const root = 'https://s3.amazonaws.com/mybucket';
+
+    // Bad, don't do this!
+    schema.path('arr').get(v => {
+      return v.map(el => Object.assign(el, { url: root + el.url }));
+    });
+
+    // Later
+    doc.arr.push({ key: String });
+    doc.arr[0]; // 'undefined' because every `doc.arr` creates a new array!
+    ```
+
+    Instead of declaring a getter on the array as shown above, you should
+    declare a getter on the `url` string as shown below. If you need to declare
+    a getter on a nested document or array, be very careful!
+
+    ```javascript
+    const schema = new Schema({
+      arr: [{ url: String }]
+    });
+
+    const root = 'https://s3.amazonaws.com/mybucket';
+
+    // Good, do this instead of declaring a getter on `arr`
+    schema.path('arr.0.url').get(v => `${root}${v}`);
+    ```
+
     <h3 id="customtypes"><a href="#customtypes">Creating Custom Types</a></h3>
 
     Mongoose can also be extended with custom SchemaTypes. Search the