Cleanup: improve 'view3d_opengl_read_Z_pixels' comment

The comment implies that the operation is slow because it creates and
destroys a framebuffer.

Actually the slowness comes from forcing CPU-GPU synchronization.

source/blender/editors/space_view3d/view3d_draw.cc

@@ -2219,8 +2219,8 @@ static void validate_object_select_id(Depsgraph *depsgraph,
   v3d->runtime.flag |= V3D_RUNTIME_DEPTHBUF_OVERRIDDEN;
 }
 
-/* TODO: Creating, attaching texture, and destroying a framebuffer is quite slow.
- *       Calling this function should be avoided during interactive drawing. */
+/* Avoid calling this function multiple times in sequence to prevent frequent CPU-GPU
+ * synchronization (which can be very slow). */
 static void view3d_opengl_read_Z_pixels(GPUViewport *viewport, rcti *rect, void *data)
 {
   GPUTexture *depth_tx = GPU_viewport_depth_texture(viewport);