patch 8.0.1595: no autocommand triggered before exiting

Problem:    No autocommand triggered before exiting.
Solution:   Add the ExitPre autocommand event.
diff --git a/runtime/doc/autocmd.txt b/runtime/doc/autocmd.txt
index 7c547df..e08a7cb 100644
--- a/runtime/doc/autocmd.txt
+++ b/runtime/doc/autocmd.txt
@@ -285,7 +285,8 @@
 |GUIFailed|		after starting the GUI failed
 |TermResponse|		after the terminal response to |t_RV| is received
 
-|QuitPre|		when using `:quit`, before deciding whether to quit
+|QuitPre|		when using `:quit`, before deciding whether to exit
+|ExitPre|		when using a command that may make Vim exit
 |VimLeavePre|		before exiting Vim, before writing the viminfo file
 |VimLeave|		before exiting Vim, after writing the viminfo file
 
@@ -651,6 +652,11 @@
 					"auto"   to trigger on 'autochdir'.
 					"drop"	 to trigger on editing a file
 				<afile> is set to the new directory name.
+							*ExitPre*
+ExitPre				When using `:quit`, `:wq` in a way it makes
+				Vim exit, or using `:qall`, just after
+				|QuitPre|.  Can be used to close any
+				non-essential window.
 							*FileChangedShell*
 FileChangedShell		When Vim notices that the modification time of
 				a file has changed since editing started.
@@ -866,6 +872,7 @@
 				or quits Vim.  Can be used to close any
 				non-essential window if the current window is
 				the last ordinary window.
+				Also see |ExitPre|.
 							*RemoteReply*
 RemoteReply			When a reply from a Vim that functions as
 				server was received |server2client()|.  The