doc: update doc for mixed:N batch size (deepmodeling#3780)

deepmodeling#3474

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## Summary by CodeRabbit

- **Documentation**
- Updated the description of the "mixed:N" string option in the training
data arguments to reflect that the "se_atten" descriptor is now
supported only for the Tensorflow backend.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

deepmd/utils/argcheck.py

@@ -2064,7 +2064,7 @@ def training_data_args():  # ! added by Ziyao: new specification style for data
 - int: all {link_sys} use the same batch size.\n\n\
 - string "auto": automatically determines the batch size so that the batch_size times the number of atoms in the system is no less than 32.\n\n\
 - string "auto:N": automatically determines the batch size so that the batch_size times the number of atoms in the system is no less than N.\n\n\
-- string "mixed:N": the batch data will be sampled from all systems and merged into a mixed system with the batch size N. Only support the se_atten descriptor.\n\n\
+- string "mixed:N": the batch data will be sampled from all systems and merged into a mixed system with the batch size N. Only support the se_atten descriptor for Tensorflow backend.\n\n\
 If MPI is used, the value should be considered as the batch size per task.'
     doc_auto_prob_style = 'Determine the probability of systems automatically. The method is assigned by this key and can be\n\n\
 - "prob_uniform"  : the probability all the systems are equal, namely 1.0/self.get_nsystems()\n\n\