doc: clarify fs.watch() and inodes on linux, os x

On Linux and OS X systems, `fs.watch()` resolves the watched path to an
inode. This clarifies that `fs.watch()` watches the inode and not the
path. If the inode of the path subsequently changes, `fs.watch()` will
continue watching the original inode and events for the path will no
longer be emitted. This is expected behavior.

Fixes: https://github.com/nodejs/node/issues/5039
PR-URL: https://github.com/nodejs/node/pull/6099


Reviewed-By: James M Snell <jasnell@gmail.com>