Added the note to Metadata too

Author: poliorcetics

src/libstd/fs.rs

@@ -1033,6 +1033,12 @@ impl Metadata {
     /// [`is_dir`], and will be false for symlink metadata
     /// obtained from [`symlink_metadata`].
     ///
+    /// This property means it is often more useful to use `!file_type.is_dir()`
+    /// than `file_type.is_file()` when your goal is to read bytes from a
+    /// source: the former includes symlink and pipes when the latter does not,
+    /// meaning you will break workflows like `diff <( prog_a ) <( prog_b )` on
+    /// a Unix-like system for example.
+    ///
     /// [`is_dir`]: struct.Metadata.html#method.is_dir
     /// [`symlink_metadata`]: fn.symlink_metadata.html
     ///