doc: add examples using an empty byteset

This doesn't change any behavior (I was wrong in #87), but instead
clarifies that empty bytesets are valid and never match anything.

Fixes #87

src/ext_slice.rs

@@ -929,14 +929,17 @@ pub trait ByteSlice: Sealed {
     /// assert_eq!(b"foo bar baz".find_byteset(b"zr"), Some(6));
     /// assert_eq!(b"foo baz bar".find_byteset(b"bzr"), Some(4));
     /// assert_eq!(None, b"foo baz bar".find_byteset(b"\t\n"));
+    /// // The empty byteset never matches.
+    /// assert_eq!(None, b"abc".find_byteset(b""));
+    /// assert_eq!(None, b"".find_byteset(b""));
     /// ```
     #[inline]
     fn find_byteset<B: AsRef<[u8]>>(&self, byteset: B) -> Option<usize> {
         byteset::find(self.as_bytes(), byteset.as_ref())
     }
 
-    /// Returns the index of the first occurrence of a byte that is not a member
-    /// of the provided set.
+    /// Returns the index of the first occurrence of a byte that is not a
+    /// member of the provided set.
     ///
     /// The `byteset` may be any type that can be cheaply converted into a
     /// `&[u8]`. This includes, but is not limited to, `&str` and `&[u8]`, but
@@ -966,6 +969,10 @@ pub trait ByteSlice: Sealed {
     /// assert_eq!(b"foo bar baz".find_not_byteset(b"fo "), Some(4));
     /// assert_eq!(b"\t\tbaz bar".find_not_byteset(b" \t\r\n"), Some(2));
     /// assert_eq!(b"foo\nbaz\tbar".find_not_byteset(b"\t\n"), Some(0));
+    /// // The negation of the empty byteset matches everything.
+    /// assert_eq!(Some(0), b"abc".find_not_byteset(b""));
+    /// // But an empty string never contains anything.
+    /// assert_eq!(None, b"".find_not_byteset(b""));
     /// ```
     #[inline]
     fn find_not_byteset<B: AsRef<[u8]>>(&self, byteset: B) -> Option<usize> {