patch 8.2.2070: can't get the exit value in VimLeave(Pre) autocommands

Problem:    Can't get the exit value in VimLeave or VimLeavePre autocommands.
Solution:   Add v:exiting like in Neovim. (Yegappan Lakshmanan, closes #7395)
diff --git a/runtime/doc/autocmd.txt b/runtime/doc/autocmd.txt
index c35e899..ea9a220 100644
--- a/runtime/doc/autocmd.txt
+++ b/runtime/doc/autocmd.txt
@@ -1214,6 +1214,7 @@
 				To detect an abnormal exit use |v:dying|.
 				When v:dying is 2 or more this event is not
 				triggered.
+				To get the exit code use |v:exiting|.
 							*VimLeavePre*
 VimLeavePre			Before exiting Vim, just before writing the
 				.viminfo file.  This is executed only once,
@@ -1224,6 +1225,7 @@
 <				To detect an abnormal exit use |v:dying|.
 				When v:dying is 2 or more this event is not
 				triggered.
+				To get the exit code use |v:exiting|.
 							*VimResized*
 VimResized			After the Vim window was resized, thus 'lines'
 				and/or 'columns' changed.  Not when starting