Add information about <topic> system paramter

Adding some API docs for changes from #1076

Signed-off-by: Kenji Brameld <kenjibrameld@gmail.com>

src/systems/joint_state_publisher/JointStatePublisher.hh

@@ -35,14 +35,17 @@ namespace systems
 {
  /// \brief The JointStatePub system publishes state information for
  /// a model. The published message type is ignition::msgs::Model, and the
- /// publication topic is "/world/<world_name>/model/<model_name>/state".
+ /// publication topic is determined by the `<topic>` parameter.
  ///
  /// By default the JointStatePublisher will publish all joints for
  /// a model. Use the `<joint_name>` system parameter, described below, to
  /// control which joints are published.
  ///
  /// # System Parameters
  ///
+ /// `<topic>`: Name of the topic to publish to. This parameter is optional,
+ /// and if not provided, the joint state will be published to
+ /// "/world/<world_name>/model/<model_name>/state".
  /// `<joint_name>`: Name of a joint to publish. This parameter can be
  /// specified multiple times, and is optional. All joints in a model will
  /// be published if joint names are not specified.