patch 9.1.0762: 'cedit', 'termwinkey' and 'wildchar' may not be parsed correctly

Problem:  'cedit', 'termwinkey' and 'wildchar' may not be parsed
          correctly
Solution: improve string_to_key() function in option.c
          (Milly)

- Problem: `^@` raises an error.
  Solution: Store as `<Nul>`.
- Problem: `<t_xx` does not raise an error.
  Solution: Raise an error if closing `>` is missing.
- Problem: Single `<` or `^` raises an error. It is inconvenient for users.
  Solution: They are stored as a single character.

closes: #15811

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 10a0680..d1a0691 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 Oct 02
+*options.txt*	For Vim version 9.1.  Last change: 2024 Oct 06
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -8432,8 +8432,11 @@
 			local to window
 	The key that starts a CTRL-W command in a terminal window.  Other keys
 	are sent to the job running in the window.
-	The <> notation can be used, e.g.: >
-		:set termwinkey=<C-L>
+	The key can be specified as a single character, a |key-notation| (e.g.
+	<Up>, <C-F>) or a letter preceded with a caret (e.g. `^F` is CTRL-F): >
+		:set twk=X
+		:set twk=^I
+		:set twk=<C-L>
 <	The string must be one key stroke but can be multiple bytes.
 	When not set CTRL-W is used, so that CTRL-W : gets you to the command
 	line.  If 'termwinkey' is set to CTRL-L then CTRL-L : gets you to the
@@ -9330,7 +9333,12 @@
 	Some keys will not work, such as CTRL-C, <CR> and Enter.
 	<Esc> can be used, but hitting it twice in a row will still exit
 	command-line as a failsafe measure.
-	Although 'wc' is a number option, you can set it to a special key: >
+	Although 'wc' is a number option, it can be specified as a number, a
+	single character, a |key-notation| (e.g. <Up>, <C-F>) or a letter
+	preceded with a caret (e.g. `^F` is CTRL-F): >
+		:set wc=27
+		:set wc=X
+		:set wc=^I
 		:set wc=<Tab>
 <	NOTE: This option is set to the Vi default value when 'compatible' is
 	set and to the Vim default value when 'compatible' is reset.