runtime(doc): improve typedchar documentation for KeyInputPre autocmd

closes: #15521

Signed-off-by: Christian Brabandt <cb@256bit.org>
Signed-off-by: Shougo Matsushita <Shougo.Matsu@gmail.com>

Author: Shougo

runtime/doc/autocmd.txt

@@ -1,4 +1,4 @@
-*autocmd.txt*   For Vim version 9.1.  Last change: 2024 Aug 12
+*autocmd.txt*   For Vim version 9.1.  Last change: 2024 Aug 18
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -992,7 +992,10 @@ KeyInputPre			Just before a key is processed after mappings
 				character is used.
 				The following values of |v:event| are set:
 				   typed	The key is typed or not.
-				   typedchar	The (actual) typed key.
+				   typedchar	The (actual) typed key since
+						the last |KeyInputPre| call.
+				Note: "typedchar" may be empty if successive
+				|KeyInputPre| autocmds are processed.
 				It is not allowed to change the text
 				|textlock| or the current mode.
 				{only with the +eval feature}