[Docs] Add docs to NVTX options

### 📚 The doc issue

when building lmdeploy from `build_all_wheel.sh`, by default the NVTX option is `ON` at compile time. However, you also need to set environment variable `FT_NVTX` to `ON` so that the `is_enable_ft_nvtx` can be set to `true` and then `nvtx push` and `nvtx pop` are actually triggered.

### Suggest a potential alternative/fix

Maybe just add some comments in `entrypoint_build.sh` or `CMakeLists.txt` to let users know : )