patch 9.1.0752: can set 'cedit' to an invalid value

Problem:  can set cedit to an invalid value
Solution: Check that the value is a valid key name
          (Milly)

closes: #15778

Signed-off-by: Milly <milly.ca@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 94671d1..49087d3 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -1,4 +1,4 @@
-*options.txt*	For Vim version 9.1.  Last change: 2024 Sep 26
+*options.txt*	For Vim version 9.1.  Last change: 2024 Oct 01
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -1651,9 +1651,10 @@
 	The default is CTRL-F when 'compatible' is off.
 	Only non-printable keys are allowed.
 	The key can be specified as a single character, but it is difficult to
-	type.  The preferred way is to use the <> notation.  Examples: >
-		:exe "set cedit=\<C-Y>"
-		:exe "set cedit=\<Esc>"
+	type.  The preferred way is to use |key-notation| (e.g. <Up>, <C-F>) or
+	a letter preceded with a caret (e.g. `^F` is CTRL-F).  Examples: >
+		:set cedit=^Y
+		:set cedit=<Esc>
 <	|Nvi| also has this option, but it only uses the first character.
 	See |cmdwin|.
 	NOTE: This option is set to the Vim default value when 'compatible'