[stdlib] Change post-nil guarantee for missed case of IteratorType.next()

Author: PatrickPijnappel

stdlib/public/core/Filter.swift

@@ -18,12 +18,11 @@
 public struct LazyFilterIterator<
   Base : IteratorProtocol
 > : IteratorProtocol, Sequence {
-  /// Advances to the next element and returns it, or `nil` if no next
-  /// element exists.
+  /// Advance to the next element and return it, or `nil` if no next element
+  /// exists.  Once `nil` has been returned, all subsequent calls return `nil`.
   ///
   /// - Precondition: `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`.
+  ///   since the copy was made.
   public mutating func next() -> Base.Element? {
     while let n = _base.next() {
       if _predicate(n) {