Added a description over the function.

lib/pipeline/documentSources/UnwindDocumentSource.js

@@ -196,6 +196,12 @@ proto.unwindPath = function unwindPath(fieldPath) {
 	this._unwinder = new klass.Unwinder(fieldPath);
 };
 
+/**
+ * Serialize the data associated with the unwind path.
+ *
+ * @param builder
+ * @param explain
+ */
 proto.serialize = function serialize(builder, explain) {
 	if (!this._unwindPath) throw new Error("unwind path does not exist!");
 	builder[this.getSourceName()] = this._unwindPath.getPath(true);