docs: Update JAX backend documentation on training support (#5482)

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## Summary by CodeRabbit

* **Documentation**
  * Updated backend documentation by refining content clarity.

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Signed-off-by: Jinzhe Zeng <jinzhe.zeng@ustc.edu.cn>

Author: njzjz

doc/backend.md

@@ -33,7 +33,6 @@ Both `.xlo` and `.jax` are customized format extensions defined in DeePMD-kit, s
 `.savedmodel` is the TensorFlow [SavedModel format](https://www.tensorflow.org/guide/saved_model) generated by [JAX2TF](https://www.tensorflow.org/guide/jax2tf), which needs the installation of TensorFlow.
 Only the `.savedmodel` format supports C++ inference, which needs the TensorFlow C++ interface.
 The model is device-specific, so that the model generated on the GPU device cannot be run on the CPUs.
-Currently, this backend is developed actively, and has no support for training.
 
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