Merge pull request #3839 from jtbandes/se-0133-3

[stdlib] Update doc comments for SE-0133

Author: gribozavr

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

@@ -623,7 +623,7 @@ extension Sequence {
   ///
   /// - 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}] {