Minor fixes (erincatto/box2d#999)

erincatto/box2d#993
erincatto/box2d#992

Author: ikpil

src/Box2D.NET/B2Joints.cs

@@ -75,6 +75,8 @@ public static b2FilterJointDef b2DefaultFilterJointDef()
             return def;
         }
 
+        /// Use this to initialize your joint definition
+        /// @ingroup prismatic_joint
         public static B2PrismaticJointDef b2DefaultPrismaticJointDef()
         {
             B2PrismaticJointDef def = new B2PrismaticJointDef();