comment why FSEventStreamFlushSync is necessary
comment why FSEventStreamFlushSync is necessary

Summary:
We have some confirmation FSEventStreamFlushSync is at least part of
the story in fixing the incorrect query results on macOS. Encode in
comments why.

Reviewed By: xavierd

Differential Revision: D29050360

fbshipit-source-id: 35ec005006ad251c31b7692922cce5322dadd3a2

nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman