[Enhancement] Unify the naming style of operators

[Redempt1onzzZZ]

The name style of mlx.nn.hard_shrink and mlx.nn.softshrink would be better if unified.

[Redempt1onzzZZ]

Besides, I've noticed that many operators' documentation is incomplete or has inconsistent formats, like nn.Softmin and nn.softmax. If you need, I can spend some effort here.