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)

runtime/doc/autocmd.txt

@@ -1214,6 +1214,7 @@ VimLeave			Before exiting Vim, just after writing the
 				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 @@ VimLeavePre			Before exiting Vim, just before writing the
 <				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

runtime/doc/eval.txt

@@ -1850,6 +1850,13 @@ v:dying		Normally zero.  When a deadly signal is caught it's set to
 <		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|.