[API Compatibility No.191] Update poisson_nll_loss docs for target/eps alias support -part

[Aidenwu0209]

Summary

• Update poisson_nll_loss_cn.rst: add alias annotations (别名：``target``` for label, 别名：``eps``` for epsilon), fix parameter name from `eps` to `epsilon` to match function signature
• Update torch.nn.functional.poisson_nll_loss.md: update parameter mapping remarks to note Paddle now supports target and eps as aliases

Related PRs

• Paddle: [API Compatibility No.191] torch.nn.functional.poisson_nll_loss -> paddle.nn.functional.poisson_nll_loss -part Paddle#77919
• PaConvert: [API Compatibility No.191] Remove kwargs_change for poisson_nll_loss since Paddle now accepts target/eps aliases -part PaConvert#837

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User Experience

PR Types

New features

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