fix(document): add parent() function to subdocument to match array su…

…bdoc

Fix #5134

lib/types/subdocument.js

@@ -102,6 +102,16 @@ Subdocument.prototype.ownerDocument = function() {
   return this.$__.ownerDocument;
 };
 
+/**
+ * Returns this sub-documents parent document.
+ *
+ * @api public
+ */
+
+Subdocument.prototype.parent = function() {
+  return this.$parent;
+};
+
 /**
  * Null-out this subdoc
  *