Explain how watch does not follow moves or renames

This summarises the knowledge gathered from notify-rs#165 (comment)

Closes: notify-rs#165

src/lib.rs

@@ -592,6 +592,12 @@ pub trait Watcher: Sized {
     ///
     /// If the `path` is a file, `recursive_mode` will be ignored and events will be delivered only
     /// for the file.
+    ///
+    /// If the `path` is renamed (moved) or deleted, it will automatically be unwatched.  I.e. watch
+    /// does not follow moved directories or files.  If at a later point another file or directory
+    /// is moved into the same location, it will not be automatically watched.  If you need that
+    /// behaviour, it is recommended that you also watch the parent directory non-recursively and
+    /// implement an opportunistic watcher yourself.
     fn watch<P: AsRef<Path>>(&mut self, path: P, recursive_mode: RecursiveMode) -> Result<()>;
 
     /// Stop watching a path.