Add comment on _StateMachine.

Author: inamiy

SwiftTask/_StateMachine.swift

@@ -8,8 +8,12 @@
 
 import Foundation
 
+///
 /// fast, naive event-handler-manager in replace of ReactKit/SwiftState (dynamic but slow),
 /// introduced from SwiftTask 2.6.0
+///
+/// see also: https://github.com/ReactKit/SwiftTask/pull/22
+///
 internal class _StateMachine<Progress, Value, Error>
 {
     internal typealias ErrorInfo = Task<Progress, Value, Error>.ErrorInfo
@@ -36,6 +40,7 @@ internal class _StateMachine<Progress, Value, Error>
     {
         let oldProgress = self.progress
 
+        // NOTE: if `weakified = false`, don't store progressValue for less memory footprint
         if !self.weakified {
             self.progress = progress
         }