HDKeys: Add jsdocs to hdprivatekey

lib/hdprivatekey.js

@@ -480,14 +480,30 @@ HDPrivateKey.prototype.toObject = function toObject() {
   };
 };
 
+/**
+ * Returns a JSON representation of the HDPrivateKey
+ *
+ * @return {string}
+ */
 HDPrivateKey.prototype.toJSON = function toJSON() {
   return JSON.stringify(this.toObject());
 };
 
+/**
+ * Build a HDPrivateKey from a buffer
+ *
+ * @param {Buffer} arg
+ * @return {HDPrivateKey}
+ */
 HDPrivateKey.fromBuffer = function(arg) {
   return new HDPrivateKey(arg);
 };
 
+/**
+ * Returns a buffer representation of the HDPrivateKey
+ *
+ * @return {string}
+ */
 HDPrivateKey.prototype.toBuffer = function() {
   return this._buffers.xprivkey;
 };