add note to lines docs about empty str behavior

Author: msmoiz

library/core/src/str/mod.rs

@@ -1251,6 +1251,8 @@ impl str {
     /// ending will return the same lines as an otherwise identical string
     /// without a final line ending.
     ///
+    /// An empty string returns no lines.
+    ///
     /// # Examples
     ///
     /// Basic usage:
@@ -1281,6 +1283,15 @@ impl str {
     ///
     /// assert_eq!(None, lines.next());
     /// ```
+    ///
+    /// An empty string returns no lines:
+    ///
+    /// ```
+    /// let text = "";
+    /// let mut lines = text.lines();
+    ///
+    /// assert_eq!(lines.next(), None);
+    /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn lines(&self) -> Lines<'_> {