* fix some doc bug test=document_fix

* fix some docs issues, test=document_fix

* beta -> \beta in softplus

* threshold -> \varepsilon in softplus

* parameter name

* delta -> \delta in smooth_l1_loss

* fix some docs test=document_fix

* fix docs test=document_fix

* fix docs && 增加空行 test=document_fix

* Update python/paddle/nn/functional/activation.py, test=document_fix

* Update python/paddle/nn/layer/activation.py, test=document_fix
Co-authored-by: NSigureMo <sigure.qaq@gmail.com>