export asyncdispatch handles (#15140)

* improve epoll docs

* export handles

* add comments and changelog

changelog.md

@@ -143,6 +143,8 @@
   `CountTable.inc` takes `val: int` again not `val: Positive`; I.e. it can "count down" again.
 - Removed deprecated symbols from `macros` module, deprecated as far back as `0.15`.
 
+- Export `asyncdispatch.PDispatcher.handles` so that an external library can register them.
+
 
 ## Language changes
 - The `=destroy` hook no longer has to reset its target, as the compiler now automatically inserts

lib/pure/asyncdispatch.nim

@@ -253,7 +253,7 @@ when defined(windows) or defined(nimdoc):
 
     PDispatcher* = ref object of PDispatcherBase
       ioPort: Handle
-      handles: HashSet[AsyncFD]
+      handles*: HashSet[AsyncFD] # Export handles so that an external library can register them.
 
     CustomObj = object of OVERLAPPED
       data*: CompletionData