Indicate why str::{get,get_mut} examples return None.

src/liballoc/str.rs

@@ -328,11 +328,16 @@ pub fn as_ptr(&self) -> *const u8 {
     /// # Examples
     ///
     /// ```
-    /// let v = "🗻∈🌏";
-    /// assert_eq!(Some("🗻"), v.get(0..4));
-    /// assert!(v.get(1..).is_none());
-    /// assert!(v.get(..8).is_none());
-    /// assert!(v.get(..42).is_none());
+    /// let mut v = String::from("🗻∈🌏");
+    ///
+    /// assert_eq!(Some("🗻"), v.get(0..4);
+    ///
+    /// // indices not on UTF-8 sequence boundaries
+    /// assert!(v.get_mut(1..).is_none());
+    /// assert!(v.get_mut(..8).is_none());
+    ///
+    /// // out of bounds
+    /// assert!(v.get_mut(..42).is_none());
     /// ```
     #[stable(feature = "str_checked_slicing", since = "1.20.0")]
     #[inline]
@@ -351,9 +356,14 @@ pub fn get<I: SliceIndex<str>>(&self, i: I) -> Option<&I::Output> {
     ///
     /// ```
     /// let mut v = String::from("🗻∈🌏");
+    ///
     /// assert_eq!(Some("🗻"), v.get_mut(0..4).map(|v| &*v));
+    ///
+    /// // indices not on UTF-8 sequence boundaries
     /// assert!(v.get_mut(1..).is_none());
     /// assert!(v.get_mut(..8).is_none());
+    ///
+    /// // out of bounds
     /// assert!(v.get_mut(..42).is_none());
     /// ```
     #[stable(feature = "str_checked_slicing", since = "1.20.0")]