Fix is_char_boundary documentation

Author: Rantanen

src/libcore/str/mod.rs

@@ -2270,12 +2270,11 @@ impl str {
         self.len() == 0
     }
 
-    /// Checks that `index`-th byte lies at the start and/or end of a
-    /// UTF-8 code point sequence.
+    /// Checks that `index`-th byte is the first byte in a UTF-8 code point
+    /// sequence or the end of the string.
     ///
     /// The start and end of the string (when `index == self.len()`) are
-    /// considered to be
-    /// boundaries.
+    /// considered to be boundaries.
     ///
     /// Returns `false` if `index` is greater than `self.len()`.
     ///