remove all mention of NULL in doc.

Author: jeremyletang

src/rsfml/graphics/convex_shape.rs

@@ -468,7 +468,6 @@ impl<'self> ConvexShape<'self> {
     * a pointer to the one that you passed to this function.
     * If the source texture is destroyed and the shape tries to
     * use it, the behaviour is undefined.
-    * texture can be NULL to disable texturing.
     * If reset_rect is true, the TextureRect property of
     * the shape is automatically adjusted to the size of the new
     * texture. If it is false, the texture rect is left unchanged.

src/rsfml/graphics/image.rs

@@ -136,7 +136,8 @@ impl Image {
     *
     * # Arguments
     * * filename - Path of the image file to load
-    * Return a new Image object, or NULL if it failed
+    *
+    * Return a new Some(Image) object, or None if it failed
     */
     #[fixed_stack_segment] #[inline(never)]
     pub fn new_from_file(filename : ~str) -> Option<Image> {

src/rsfml/graphics/rectangle_shape.rs

@@ -513,7 +513,6 @@ impl<'self> RectangleShape<'self> {
     * a pointer to the one that you passed to this function.
     * If the source texture is destroyed and the shape tries to
     * use it, the behaviour is undefined.
-    * texture can be NULL to disable texturing.
     * If reset_rect is true, the TextureRect property of
     * the shape is automatically adjusted to the size of the new
     * texture. If it is false, the texture rect is left unchanged.

src/rsfml/graphics/shader.rs

@@ -121,7 +121,7 @@ impl<'self> Shader<'self> {
     * Load both the vertex and fragment shaders from source codes in memory
     *
     * This function can load both the vertex and the fragment
-    * shaders, or only one of them: pass NULL if you don't want to load
+    * shaders, or only one of them: pass None if you don't want to load
     * either the vertex shader or the fragment shader.
     * The sources must be valid shaders in GLSL language. GLSL is
     * a C-like language dedicated to OpenGL shaders; you'll