lib/loopback: fix jsdoc comments

Use
     @property {Object} [properties]
instead of
     @property {Object=} properties
for optional properties.

Use `**example**` instead of `@example`, since strong-docs don't support
the latter.

lib/loopback.js

@@ -210,7 +210,7 @@ loopback.createModel = function (name, properties, options) {
 /**
  * Create a model as described by the configuration object.
  *
- * @example
+ * **Example**
  *
  * ```js
  * loopback.createModelFromConfig({
@@ -230,8 +230,8 @@ loopback.createModel = function (name, properties, options) {
  *
  * @options {Object} model configuration
  * @property {String} name Unique name.
- * @property {Object=} properties Model properties
- * @property {Object=} options Model options. Options can be specified on the
+ * @property {Object} [properties] Model properties
+ * @property {Object} [options] Model options. Options can be specified on the
  * top level config object too. E.g. `{ base: 'User' }` is the same as
  * `{ options: { base: 'User' } }`.
  */
@@ -270,7 +270,7 @@ function buildModelOptionsFromConfig(config) {
  * @param {Model} ModelCtor The model constructor to alter.
  * @options {Object} Additional configuration to apply
  * @property {DataSource} dataSource Attach the model to a dataSource.
- * @property {Object} relations Model relations to add/update.
+ * @property {Object} [relations] Model relations to add/update.
  */
 loopback.configureModel = function(ModelCtor, config) {
   var settings = ModelCtor.settings;