[Standard library] Duplicate subscript requirement in RandomAccessCollection.

Similar to what we did for BidirectionalCollection, duplicate the
subscript-returning-SubSequence requirement in
RandomAccessCollection.

Author: DougGregor

stdlib/public/core/RandomAccessCollection.swift

@@ -81,6 +81,29 @@ public protocol RandomAccessCollection :
   ///     }
   ///     // c == MyFancyCollection([2, 4, 6, 8, 10])
   var indices: Indices { get }
+
+  /// Accesses a contiguous subrange of the collection's elements.
+  ///
+  /// The accessed slice uses the same indices for the same elements as the
+  /// original collection uses. Always use the slice's `startIndex` property
+  /// instead of assuming that its indices start at a particular value.
+  ///
+  /// This example demonstrates getting a slice of an array of strings, finding
+  /// the index of one of the strings in the slice, and then using that index
+  /// in the original array.
+  ///
+  ///     let streets = ["Adams", "Bryant", "Channing", "Douglas", "Evarts"]
+  ///     let streetsSlice = streets[2 ..< streets.endIndex]
+  ///     print(streetsSlice)
+  ///     // Prints "["Channing", "Douglas", "Evarts"]"
+  ///
+  ///     let index = streetsSlice.index(of: "Evarts")    // 4
+  ///     print(streets[index!])
+  ///     // Prints "Evarts"
+  ///
+  /// - Parameter bounds: A range of the collection's indices. The bounds of
+  ///   the range must be valid indices of the collection.
+  subscript(bounds: Range<Index>) -> SubSequence { get }
 }
 
 /// Supply the default "slicing" `subscript` for `RandomAccessCollection`