Added documentation and where clause for API uniqued()

Sources/Algorithms/Unique.swift

@@ -15,7 +15,16 @@
 
 extension Sequence where Element: Hashable {
   /// Returns an array with only the unique elements of this sequence, in the
-  /// order of the first occurence of each unique element.
+  /// order of the first occurrence of each unique element.
+  ///
+  ///     let animals = ["dog", "pig", "cat", "ox", "dog", "cat"]
+  ///     let uniqued = animals.uniqued()
+  ///     print(uniqued)
+  ///     // Prints '["dog", "pig", "cat", "ox"]'
+  ///
+  /// - Returns: An array with only the unique elements of this sequence.
+  ///  .
+  /// - Complexity: O(*n*), where *n* is the length of the sequence.
   @inlinable
   public func uniqued() -> [Element] {
     uniqued(on: { $0 })
@@ -24,8 +33,26 @@ extension Sequence where Element: Hashable {
 
 extension Sequence {
   /// Returns an array with the unique elements of this sequence (as determined
-  /// by the given projection), in the order of the first occurence of each
+  /// by the given projection), in the order of the first occurrence of each
   /// unique element.
+  ///
+  /// This example finds the elements of the `animals` array with unique
+  /// first characters:
+  ///
+  ///     let animals = ["dog", "pig", "cat", "ox", "cow", "owl"]
+  ///     let uniqued = animals.uniqued(on: {$0.first})
+  ///     print(uniqued)
+  ///     // Prints '["dog", "pig", "cat", "ox"]'
+  ///
+  /// - Parameter projection: A closure that transforms an element into the
+  ///   value to use for uniqueness. If `projection` returns the same value
+  ///   for two different elements, the second element will be excluded
+  ///   from the resulting array.
+  ///
+  /// - Returns: An array with only the unique elements of this sequence, as
+  ///   determined by the result of `projection` for each element.
+  ///
+  /// - Complexity: O(*n*), where *n* is the length of the sequence.
   @inlinable
   public func uniqued<Subject: Hashable>(
     on projection: (Element) throws -> Subject