patch 8.2.3591: no event is triggered when closing a window

Problem:    No event is triggered when closing a window.
Solution:   Add the WinClosed event. (Naohiro Ono, closes #9110)
diff --git a/runtime/doc/autocmd.txt b/runtime/doc/autocmd.txt
index 6a4edf9..fdac7da 100644
--- a/runtime/doc/autocmd.txt
+++ b/runtime/doc/autocmd.txt
@@ -348,6 +348,7 @@
 
 |WinNew|		after creating a new window
 |TabNew|		after creating a new tab page
+|WinClosed|		after closing a window
 |TabClosed|		after closing a tab page
 |WinEnter|		after entering another window
 |WinLeave|		before leaving a window
@@ -1280,6 +1281,12 @@
 VimSuspend			When the Vim instance is suspended.  Only when
 				CTRL-Z was typed inside Vim, not when the
 				SIGSTOP or SIGTSTP signal was sent to Vim.
+							*WinClosed*
+WinClosed			After closing a window.  The pattern is
+				matched against the |window-ID|.  Both
+				<amatch> and <afile> are set to the
+				|window-ID|.  Non-recursive (event cannot
+				trigger itself).
 							*WinEnter*
 WinEnter			After entering another window.  Not done for
 				the first window, when Vim has just started.