[stdlib] Update doc comments for SE-0133

Author: jtbandes

stdlib/public/core/Join.swift

@@ -168,7 +168,7 @@ extension Sequence where Iterator.Element : Sequence {
   ///   sequence's elements.
   /// - Returns: The joined sequence of elements.
   ///
-  /// - SeeAlso: `flatten()`
+  /// - SeeAlso: `joined()`
   public func joined<Separator : Sequence>(
     separator: Separator
   ) -> JoinedSequence<Self>

stdlib/public/core/SequenceAlgorithms.swift.gyb

@@ -615,15 +615,15 @@ extension Sequence {
   ///     // [1, 2, 2, 3, 3, 3, 4, 4, 4, 4]
   ///
   /// In fact, `s.flatMap(transform)`  is equivalent to
-  /// `Array(s.map(transform).flatten())`.
+  /// `Array(s.map(transform).joined())`.
   ///
   /// - Parameter transform: A closure that accepts an element of this
   ///   sequence as its argument and returns a sequence or collection.
   /// - Returns: The resulting flattened array.
   ///
   /// - Complexity: O(*m* + *n*), where *m* is the length of this sequence
   ///   and *n* is the length of the result.
-  /// - SeeAlso: `flatten()`, `map(_:)`
+  /// - SeeAlso: `joined()`, `map(_:)`
   public func flatMap<SegmentOfResult : Sequence>(
     _ transform: @noescape (${GElement}) throws -> SegmentOfResult
   ) rethrows -> [SegmentOfResult.${GElement}] {