Add more doc tests

Author: EbbDrop

library/core/src/iter/traits/iterator.rs

@@ -4041,6 +4041,12 @@ pub trait Iterator {
     ///
     /// assert!([1, 2, 2, 9].iter().is_sorted_by(|a, b| a <= b));
     /// assert!(![1, 2, 2, 9].iter().is_sorted_by(|a, b| a < b));
+    ///
+    /// assert!([0].iter().is_sorted_by(|a, b| true));
+    /// assert!([0].iter().is_sorted_by(|a, b| false));
+    ///
+    /// assert!(std::iter::empty::<i32>().is_sorted_by(|a, b| false));
+    /// assert!(std::iter::empty::<i32>().is_sorted_by(|a, b| true));
     /// ```
     #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
     #[rustc_do_not_const_check]

library/core/src/slice/mod.rs

@@ -4118,6 +4118,13 @@ impl<T> [T] {
     ///
     /// assert!([1, 2, 2, 9].is_sorted_by(|a, b| a <= b));
     /// assert!(![1, 2, 2, 9].is_sorted_by(|a, b| a < b));
+    ///
+    /// assert!([0].is_sorted_by(|a, b| true));
+    /// assert!([0].is_sorted_by(|a, b| false));
+    ///
+    /// let empty: [i32; 0] = [];
+    /// assert!(empty.is_sorted_by(|a, b| false));
+    /// assert!(empty.is_sorted_by(|a, b| true));
     /// ```
     #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
     #[must_use]