Make yank-transform-functions into defvar

* doc/lispref/text.texi (Yanking): Adjust.
* lisp/simple.el (yank-transform-functions): Make into defvar
because it's not that useful as a user option.

doc/lispref/text.texi

@@ -1111,7 +1111,7 @@ or specifying key bindings.  It takes effect after
 @code{yank-handled-properties}.
 @end defopt
 
-@defopt yank-transform-functions
+@defvar yank-transform-functions
 This variable is a list of functions.  Each function is called (in
 order) with the string to be yanked as the argument, and should
 return a (possibly transformed) string.  This variable can be set
@@ -1127,7 +1127,7 @@ could say something like:
 	 '(string-clean-whitespace)))
     (call-interactively #'yank)))
 @end lisp
-@end defopt
+@end defvar
 
 @node Yank Commands
 @subsection Functions for Yanking

etc/NEWS

@@ -2268,8 +2268,8 @@ This command lets you examine all data in the current selection and
 the clipboard, and insert it into the buffer.
 
 +++
-** New user option 'yank-transform-functions'.
-This function allows the user to alter the string to be inserted.
+** New variable 'yank-transform-functions'.
+This variable allows the user to alter the string to be inserted.
 
 ---
 ** New command 'yank-in-context'.

lisp/simple.el

@@ -5946,17 +5946,14 @@ See also `yank-handled-properties'."
   :group 'killing
   :version "24.3")
 
-(defcustom yank-transform-functions nil
+(defvar yank-transform-functions nil
   "Hook run on strings to be yanked.
 Each function in this list will be called (in order) with the
 string to be yanked as the sole argument, and should return the (possibly)
 transformed string.
 
 The functions will be called with the destination buffer as the current
-buffer, and with point at the place where the string is to be inserted."
-  :type 'hook
-  :version "29.1"
-  :group 'killing)
+buffer, and with point at the place where the string is to be inserted.")
 
 (defvar yank-window-start nil)
 (defvar yank-undo-function nil