docs(gpu): document depth texture limitation in SDL_CopyGPUTextureToTexture (#14693)

include/SDL3/SDL_gpu.h

@@ -3903,6 +3903,9 @@ extern SDL_DECLSPEC void SDLCALL SDL_UploadToGPUBuffer(
  * This copy occurs on the GPU timeline. You may assume the copy has finished
  * in subsequent commands.
  *
+ * This function does not support copying between depth and color textures. For
+ * those, copy the texture to a buffer and then to the destination texture.
+ *
  * \param copy_pass a copy pass handle.
  * \param source a source texture region.
  * \param destination a destination texture region.