Clarify writev() doc
Clarify writev() doc

Test Plan: fbgs for all TAsyncTransport uses and verify that no implementations rely on this.

Reviewed By: brianp@fb.com

Subscribers: alandau, bmatheny

FB internal diff: D1340948


uber/fbthrift,uber/fbthrift,jesboat/fbthrift,fengshao0907/fbthrift,fengshao0907/fbthrift,fengshao0907/fbthrift,soumith/fbthrift,SergeyMakarenko/fbthrift,sinjar666/fbthrift,maxwell-demon/fbthrift,LinusU/fbthrift,uber/fbthrift,SergeyMakarenko/fbthrift,chjp2046/fbthrift,chjp2046/fbthrift,HankFaan/fbthrift,getyourguide/fbthrift,soumith/fbthrift,LinusU/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,HankFaan/fbthrift,sinjar666/fbthrift,uber/fbthrift,kangkot/fbthrift,chjp2046/fbthrift,SergeyMakarenko/fbthrift,facebook/fbthrift,SergeyMakarenko/fbthrift,guker/fbthrift,Orvid/fbthrift,LinusU/fbthrift,getyourguide/fbthrift,raphaelamorim/fbthrift,LinusU/fbthrift,chjp2046/fbthrift,Orvid/fbthrift,guker/fbthrift,Orvid/fbthrift,soumith/fbthrift,project-zerus/fbthrift,maxwell-demon/fbthrift,fengshao0907/fbthrift,SergeyMakarenko/fbthrift,project-zerus/fbthrift,HankFaan/fbthrift,Orvid/fbthrift,jesboat/fbthrift,sinjar666/fbthrift,SergeyMakarenko/fbthrift,sinjar666/fbthrift,getyourguide/fbthrift,guker/fbthrift,fengshao0907/fbthrift,maxwell-demon/fbthrift,HankFaan/fbthrift,soumith/fbthrift,LinusU/fbthrift,uber/fbthrift,SergeyMakarenko/fbthrift,project-zerus/fbthrift,SergeyMakarenko/fbthrift,raphaelamorim/fbthrift,Orvid/fbthrift,uber/fbthrift,guker/fbthrift,HankFaan/fbthrift,sinjar666/fbthrift,Orvid/fbthrift,jesboat/fbthrift,facebook/fbthrift,soumith/fbthrift,project-zerus/fbthrift,project-zerus/fbthrift,kangkot/fbthrift,getyourguide/fbthrift,facebook/fbthrift,fengshao0907/fbthrift,facebook/fbthrift,uber/fbthrift,project-zerus/fbthrift,maxwell-demon/fbthrift,guker/fbthrift,kangkot/fbthrift,facebook/fbthrift,kangkot/fbthrift,jesboat/fbthrift,SergeyMakarenko/fbthrift,jesboat/fbthrift,soumith/fbthrift,maxwell-demon/fbthrift,guker/fbthrift,kangkot/fbthrift,fengshao0907/fbthrift,chjp2046/fbthrift,project-zerus/fbthrift,facebook/fbthrift,LinusU/fbthrift,maxwell-demon/fbthrift,raphaelamorim/fbthrift,chjp2046/fbthrift,SergeyMakarenko/fbthrift,facebook/fbthrift,project-zerus/fbthrift,guker/fbthrift,jesboat/fbthrift,fengshao0907/fbthrift,jesboat/fbthrift,maxwell-demon/fbthrift,uber/fbthrift,chjp2046/fbthrift,sinjar666/fbthrift,guker/fbthrift,sinjar666/fbthrift,LinusU/fbthrift,project-zerus/fbthrift,HankFaan/fbthrift,getyourguide/fbthrift,soumith/fbthrift,fengshao0907/fbthrift,sinjar666/fbthrift,kangkot/fbthrift,jesboat/fbthrift,LinusU/fbthrift,kangkot/fbthrift,raphaelamorim/fbthrift,soumith/fbthrift,Orvid/fbthrift,maxwell-demon/fbthrift,raphaelamorim/fbthrift,LinusU/fbthrift,jesboat/fbthrift,fengshao0907/fbthrift,getyourguide/fbthrift,kangkot/fbthrift,getyourguide/fbthrift,Orvid/fbthrift,project-zerus/fbthrift,LinusU/fbthrift,raphaelamorim/fbthrift,getyourguide/fbthrift,jesboat/fbthrift,guker/fbthrift,LinusU/fbthrift,fengshao0907/fbthrift,jesboat/fbthrift,raphaelamorim/fbthrift,jesboat/fbthrift,raphaelamorim/fbthrift,guker/fbthrift,maxwell-demon/fbthrift,raphaelamorim/fbthrift,uber/fbthrift,sinjar666/fbthrift,soumith/fbthrift,uber/fbthrift,chjp2046/fbthrift,maxwell-demon/fbthrift,HankFaan/fbthrift,uber/fbthrift,facebook/fbthrift,HankFaan/fbthrift,kangkot/fbthrift,HankFaan/fbthrift,guker/fbthrift,getyourguide/fbthrift,sinjar666/fbthrift,Orvid/fbthrift,facebook/fbthrift,kangkot/fbthrift,project-zerus/fbthrift,HankFaan/fbthrift,chjp2046/fbthrift,HankFaan/fbthrift,Orvid/fbthrift,soumith/fbthrift,kangkot/fbthrift,raphaelamorim/fbthrift,SergeyMakarenko/fbthrift,soumith/fbthrift,chjp2046/fbthrift,Orvid/fbthrift,chjp2046/fbthrift,project-zerus/fbthrift,getyourguide/fbthrift,sinjar666/fbthrift,uber/fbthrift,getyourguide/fbthrift,raphaelamorim/fbthrift,maxwell-demon/fbthrift,chjp2046/fbthrift