expand API doc

Signed-off-by: Ian Chen <ichen@openrobotics.org>

include/gz/physics/Kinematic.hh

@@ -33,11 +33,15 @@ namespace gz
       class Link : public virtual Feature::Link<PolicyT, FeaturesT>
       {
         /// \brief Set link to be kinematic.
+        /// A kinematic link does not react to forces, e.g. gravity or other dynamic
+        /// objects. It reacts to pose or velocity commands that are set on the link
+        /// (via a FreeGroup) or on the joint that connects the kinematic links.
         /// \param[i] _kinematic True to make this link kinematic.
         public: void SetKinematic(bool _kinematic);
 
         /// \brief Get whether this link is kinematic.
         /// \return True if the link is kinematic, false otherwise.
+        /// \sa SetKinematic
         public: bool GetKinematic() const;
       };