Mention --fs-reload (fsmon) in Graceful Reloading doc

[rattrayalex]

I'm a newcomer here, but it's my impression that --fs-reload (a la fsmon) has arrived since the Art of Graceful Reloading document was written.

There's strikingly little (no?) documentation of fs-reload, but it seems to perform well for graceful reloads? Perhaps it should be added to this document as a super-simple option? If it is not, in fact, a graceful reloader, perhaps that could be mentioned in that doc as well?