Call out the behavior for empty sequences.

Fixes rdar://60543202

(cherry picked from commit d0d4a88823)

stdlib/public/Concurrency/AsyncSequence.swift

@@ -236,6 +236,8 @@ extension AsyncSequence {
   /// The predicate executes each time the asynchronous sequence produces an
   /// element, until either the predicate returns `false` or the sequence ends.
   ///
+  /// If the asynchronous sequence is empty, this method returns `true`.
+  ///
   /// - Parameter predicate: A closure that takes an element of the asynchronous
   ///   sequence as its argument and returns a Boolean value that indicates
   ///   whether the passed element satisfies a condition.

stdlib/public/core/SequenceAlgorithms.swift

@@ -529,6 +529,8 @@ extension Sequence {
   ///     let allHaveAtLeastFive = names.allSatisfy({ $0.count >= 5 })
   ///     // allHaveAtLeastFive == true
   ///
+  /// If the sequence is empty, this method returns `true`.
+  ///
   /// - Parameter predicate: A closure that takes an element of the sequence
   ///   as its argument and returns a Boolean value that indicates whether
   ///   the passed element satisfies a condition.