Target Java 6 and fix a few docstrings

src/play_clj/g2d_physics.clj

@@ -53,7 +53,10 @@
      (u/call! object# ~k ~@options)))
 
 (defn add-body!
-  "Adds the `body` to the `screen` for physics simulations and returns it."
+  "Creates a body from the `b-def`, adds it to the `screen` for physics
+simulations and returns it.
+
+    (add-body! screen (body-def :dynamic))"
   [screen b-def]
   (box-2d! screen :create-body b-def))
 

src/play_clj/g3d_physics.clj

@@ -125,7 +125,9 @@
   (btSoftBodyWorldInfo.))
 
 (defn add-body!
-  "Adds the `body` to the `screen` for physics simulations and returns it."
+  "Adds the `body` to the `screen` for physics simulations and returns it.
+
+    (add-body! screen (rigid-body info))"
   [screen body]
   (cond
     (isa? (type (:object body)) btRigidBody)