docs(schema): add Schema#virtuals to docs as a public property

Fix #10829

lib/schema.js

@@ -220,6 +220,32 @@ Object.defineProperty(Schema.prototype, 'childSchemas', {
   writable: true
 });
 
+/**
+ * Object containing all virtuals defined on this schema.
+ * The objects' keys are the virtual paths and values are instances of `VirtualType`.
+ *
+ * This property is typically only useful for plugin authors and advanced users.
+ * You do not need to interact with this property at all to use mongoose.
+ *
+ * ####Example:
+ *     const schema = new Schema({});
+ *     schema.virtual('answer').get(() => 42);
+ *
+ *     console.log(schema.virtuals); // { answer: VirtualType { path: 'answer', ... } }
+ *     console.log(schema.virtuals['answer'].getters[0].call()); // 42
+ *
+ * @api public
+ * @property virtuals
+ * @memberOf Schema
+ * @instance
+ */
+
+Object.defineProperty(Schema.prototype, 'virtuals', {
+  configurable: false,
+  enumerable: true,
+  writable: true
+});
+
 /**
  * The original object passed to the schema constructor
  *