* add .rst file of 4 control flow APIs

* add Note in docs of control flow. test=develop

* refine api docs name param

* add name param refine for ifelse,round,sqrt,sin,scatter,test=develop

* refine switch doc, test=develop, test=document_preview

* add description and sample for member functions, test=develop

* refine While doc, test=develop

* add refine Switch api doc, test=develop

* refine While, delete Switch, test=develop, test=document_preview