doc: added note to fs.watchFile on previousStat

Explains the expected behavior of previousStat in fs.watchFile() when
a watched file disappears and reappears.

PR-URL: https://github.com/nodejs/node/pull/16099
Fixes: https://github.com/nodejs/node/issues/15364


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>