Add information about <topic> system paramter

Adding some API docs for changes from gazebosim#1076

Signed-off-by: Kenji Brameld <[email protected]>

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.