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/eval.txt b/runtime/doc/eval.txt
index 72a66d5..6f594ab 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -1850,6 +1850,13 @@
 <		Note: if another deadly signal is caught when v:dying is one,
 		VimLeave autocommands will not be executed.
 
+					*v:exiting* *exiting-variable*
+v:exiting	Vim exit code.  Normally zero, non-zero when something went
+		wrong.  The value is v:null before invoking the |VimLeavePre|
+		and |VimLeave| autocmds.  See |:q|, |:x| and |:cquit|.
+		Example: >
+			:au VimLeave * echo "Exit value is " .. v:exiting
+<
 					*v:echospace* *echospace-variable*
 v:echospace	Number of screen cells that can be used for an `:echo` message
 		in the last screen line before causing the |hit-enter-prompt|.