Fix async-signal related doc for ServiceHandler::shutdownServer
Fix async-signal related doc for ServiceHandler::shutdownServer

Differential Revision: D33796228

fbshipit-source-id: c4762dea683fabf2f27300ccd230fd350b503cce

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift