[stdlib] allow construction of UTF16Index from Int

...when Foundation is loaded.  Also, doc comments for its
RandomAccessIndexType conformance

Swift SVN r24637

stdlib/objc/Foundation/ExtraStringAPIs.swift

@@ -14,9 +14,29 @@
 // imported.  Making this API dependent on Foundation decouples the
 // Swift core from a UTF16 representation.
 extension String.UTF16View.Index : RandomAccessIndexType {
+  
+  /// Construct from an integer offset.
+  public init(_ offset: Int) {
+    _precondition(offset >= 0, "Negative UTF16 index offset not allowed")
+    self.init(_offset: offset)
+    // self._offset = offset
+  }
+  
+  /// Return the minimum number of applications of `successor` or
+  /// `predecessor` required to reach `other` from `self`.
+  ///
+  /// Complexity: O(1).
   public func distanceTo(x: String.UTF16View.Index) -> Int {
     return x._offset - _offset
   }
+  
+  /// Return `self` offset by `n` steps.
+  ///
+  /// :returns: If `n > 0`, the result of applying `successor` to
+  /// `self` `n` times.  If `n < 0`, the result of applying
+  /// `predecessor` to `self` `-n` times. Otherwise, `self`.
+  ///
+  /// Complexity: O(1)
   public func advancedBy(x: Int) -> String.UTF16View.Index {
     return String.UTF16View.Index(_offset: _offset + x)
   }