patch 8.2.3864: cannot disable requesting key codes from xterm

Problem:    Cannot disable requesting key codes from xterm.
Solution:   Add the 'xtermcodes' option, default on.
diff --git a/runtime/doc/term.txt b/runtime/doc/term.txt
index 8862d0a..3e90cc0 100644
--- a/runtime/doc/term.txt
+++ b/runtime/doc/term.txt
@@ -556,8 +556,15 @@
 adjust the various t_ codes.  This avoids the problem that the xterm can
 produce different codes, depending on the mode it is in (8-bit, VT102,
 VT220, etc.).  The result is that codes like <xF1> are no longer needed.
-Note: This is only done on startup.  If the xterm options are changed after
-Vim has started, the escape sequences may not be recognized anymore.
+
+One of the codes that can change is 't_Co', the number of colors.  This will
+trigger a redraw.  If this is a problem, reset the 'xtermcodes' option as
+early as possible: >
+	set noxtermcodes
+
+Note: Requesting the key codes is only done on startup.  If the xterm options
+are changed after Vim has started, the escape sequences may not be recognized
+anymore.
 
 							*xterm-true-color*
 Vim supports using true colors in the terminal (taken from |highlight-guifg|