[stdlib] Propagate GeneratorType docs to models

Swift SVN r22208

stdlib/core/CompilerProtocols.swift

@@ -39,7 +39,7 @@ public protocol BooleanType {
 /// Also, the generators must be obtained by distinct calls to the
 /// *sequence's* `generate()` method, rather than by copying.
 public protocol GeneratorType { 
-  /// The type of element to be bound to the loop variable.
+  /// The type of element generated by `self`.
   typealias Element
 
   /// Advance to the next element and return it, or `nil` if no next
@@ -170,6 +170,12 @@ public struct GeneratorSequence<
     _base = base
   }
   
+  /// Advance to the next element and return it, or `nil` if no next
+  /// element exists.
+  ///
+  /// Requires: `next()` has not been applied to a copy of `self`
+  /// since the copy was made, and no preceding call to `self.next()`
+  /// has returned `nil`.
   public mutating func next() -> G.Element? {
     return _base.next()
   }