document expected behavior for symlinks

[skybrian]

It's unclear what's supposed to happen for symlinks. We don't need to implement full symlink support, but we should say what happens when they exist.

• Is a symlink a file? A directory? Does this change depending on the symlink's target?
• Do we detect when the symlink changes? When its target changes?
• What happens when the symlink points outside the directory being watched?