docs: add description of alias option

Fix #5287, re: #5184

Author: vkarpov15

docs/guide.jade

@@ -250,7 +250,29 @@ block content
   :markdown
     Virtual property setters are applied before other validation. So the example above would still work even if the `first` and `last` name fields were required.
 
-    Only non-virtual properties work as part of queries and for field selection.
+    Only non-virtual properties work as part of queries and for field selection. Since virtuals are not stored in MongoDB, you can't query with them.
+  
+  h5#aliases Aliases
+  :markdown
+    Aliases are a particular type of virtual where the getter and setter seamlessly get and set another property. This is handy for saving network bandwidth, so you can convert a short property name stored in the database into a longer name for code readability.
+  
+  :js
+    var personSchema = new Schema({
+      n: {
+        type: String,
+        // Now accessing `name` will get you the value of `n`, and setting `n` will set the value of `name`
+        alias: 'name'
+      }
+    });
+    
+    // Setting `name` will propagate to `n`
+    var person = new Person({ name: 'Val' });
+    console.log(person); // { n: 'Val' }
+    console.log(person.toObject({ virtuals: true })); // { n: 'Val', name: 'Val' }
+    console.log(person.name); // "Val"
+    
+    person.name = 'Not Val';
+    console.log(person); // { n: 'Not Val' }
 
   h3#options Options
   :markdown

docs/schematypes.jade

@@ -112,13 +112,15 @@ block content
     * `validate: function, adds a [validator function](http://mongoosejs.com/docs/validation.html#built-in-validators) for this property
     * `get`: function, defines a custom getter for this property using [`Object.defineProperty()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty).
     * `set`: function, defines a custom setter for this property using [`Object.defineProperty()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty).
+    * `alias`: string, mongoose >= 4.10.0 only. Defines a [virtual](http://mongoosejs.com/docs/guide.html#virtuals) with the given name that gets/sets this path.
 
   :js
     var numberSchema = new Schema({
       integerOnly: {
         type: Number,
         get: v => Math.round(v),
-        set: v => Math.round(v)
+        set: v => Math.round(v),
+        alias: 'i'
       }
     });
 
@@ -127,6 +129,10 @@ block content
     var doc = new Number();
     doc.integerOnly = 2.001;
     doc.integerOnly; // 2
+    doc.i; // 2
+    doc.i = 3.001;
+    doc.integerOnly; // 3
+    doc.i; // 3
 
   h5 Indexes
   p