Improve/Update bfloat16 docs

## 📚 Documentation

We can improve the bfloat16 docs now that 1.10 is out, with also a link on how it works (how it chooses which operations to run in bf16 + the behaviour in autocast if we set to bf16).

This is motivated by https://github.com/PyTorchLightning/pytorch-lightning/pull/9049#issuecomment-949932101