patch 8.2.0158: triggering CompleteDone earlier is not backwards compatible

Problem:    Triggering CompleteDone earlier is not backwards compatible.
            (Daniel Hahler)
Solution:   Add CompleteDonePre instead.
diff --git a/runtime/doc/autocmd.txt b/runtime/doc/autocmd.txt
index 89ba575..0dc2fff 100644
--- a/runtime/doc/autocmd.txt
+++ b/runtime/doc/autocmd.txt
@@ -374,7 +374,10 @@
 
 |MenuPopup|		just before showing the popup menu
 |CompleteChanged|	after Insert mode completion menu changed
-|CompleteDone|		after Insert mode completion is done
+|CompleteDonePre|	after Insert mode completion is done, before clearing
+			info
+|CompleteDone|		after Insert mode completion is done, after clearing
+			info
 
 |User|			to be used in combination with ":doautocmd"
 
@@ -589,8 +592,8 @@
 CompleteChanged						*CompleteChanged*
 				After each time the Insert mode completion
 				menu changed.  Not fired on popup menu hide,
-				use |CompleteDone| for that.  Never triggered
-				recursively.
+				use |CompleteDonePre| or |CompleteDone| for
+				that.  Never triggered recursively.
 
 				Sets these |v:event| keys:
 				    completed_item	See |complete-items|.
@@ -606,12 +609,22 @@
 				The size and position of the popup are also
 				available by calling |pum_getpos()|.
 
+							*CompleteDonePre*
+CompleteDonePre			After Insert mode completion is done.  Either
+				when something was completed or abandoning
+				completion. |ins-completion|
+				|complete_info()| can be used, the info is
+				cleared after triggering CompleteDonePre.
+				The |v:completed_item| variable contains
+				information about the completed item.
+
 							*CompleteDone*
 CompleteDone			After Insert mode completion is done.  Either
 				when something was completed or abandoning
 				completion. |ins-completion|
-				|complete_info()| can be used, the info is
-				cleared after triggering CompleteDone.
+				|complete_info()| cannot be used, the info is
+				cleared before triggering CompleteDone.  Use
+				CompleteDonePre if you need it.
 				The |v:completed_item| variable contains
 				information about the completed item.