add doc on suffix method

Author: michiboo

Sources/Algorithms/Suffix.swift

@@ -14,6 +14,15 @@
 //===----------------------------------------------------------------------===//
 
 extension BidirectionalCollection {
+  /// Returns a subsequence containing the elements from the end until `predicate`
+  /// returns `false` and skipping the remaining elements.
+  ///
+  /// - Parameter predicate: A closure that takes an element of the
+  ///   sequence as its argument and returns `true` if the element should
+  ///   be included or `false` if it should be excluded. Once the predicate
+  ///   returns `false` it will not be called again.
+  ///
+  /// - Complexity: O(*n*), where *n* is the length of the collection.
   public func suffix(
     while predicate: (Element) throws -> Bool
   ) rethrows -> SubSequence {