str: Correct documentation on is_char_boundary

Documentation claims it panics on out of bounds -- it regards out of
bounds as just not a char boundary.

core::str module is aware of how it works and uses it appropriately.
Maybe we should rename it to `is_valid_index`, `is_slicable_index`, or
something similar.

src/libcollections/str.rs

@@ -483,9 +483,7 @@ impl str {
     /// considered to be
     /// boundaries.
     ///
-    /// # Panics
-    ///
-    /// Panics if `index` is greater than `self.len()`.
+    /// Returns `false` if `index` is greater than `self.len()`.
     ///
     /// # Examples
     ///