Add warnings about disposing from memory

src/play_clj/core_basics.clj

@@ -141,6 +141,8 @@
 
 (defmacro sound
   "Returns a [Sound](http://libgdx.badlogicgames.com/nightlies/docs/api/com/badlogic/gdx/audio/Sound.html).
+When no longer needed, you must call `(sound! object :dispose)` to free it from
+memory.
 
     (sound \"playerhurt.wav\")
     (sound \"playerhurt.wav\" :play)"

src/play_clj/core_graphics.clj

@@ -2,6 +2,8 @@
 
 (defmacro pixmap
   "Returns a [Pixmap](http://libgdx.badlogicgames.com/nightlies/docs/api/com/badlogic/gdx/graphics/Pixmap.html).
+When no longer needed, you must call `(pixmap! object :dispose)` to free it from
+memory.
 
     (pixmap \"image.png\")"
   [& args]
@@ -10,7 +12,9 @@
       `(Pixmap. ~@args)))
 
 (defmacro pixmap!
-  "Calls a single method on a `pixmap`."
+  "Calls a single method on a `pixmap`.
+
+    (pixmap! object :dispose)"
   [object k & options]
   `(u/call! ^Pixmap ~object ~k ~@options))