bug#81547: 31.0.90; Defcustom's docstring does not mention new `BUFFER-LOCAL' optional argument for :set function

According to NEWS:

---
** New macro 'setopt-local'.
This is the buffer-local version of 'setopt' for user options rather
than plain variables and uses 'custom-set'/'set-local' to set variable
values.  A new argument, BUFFER-LOCAL, is passed to 'custom-set'
functions to indicate the buffer-local context.
---

However, this is currently `defcustom' docstring regarding `:set':

---
:set	VALUE should be a function to set the value of the symbol
	when using the Customize user interface.  It takes two arguments,
	the symbol to set and the value to give it.  The function should
	not modify its value argument destructively.  The default choice
	of function is `set-default-toplevel-value'.  If this keyword is
	defined, modifying the value of SYMBOL via `setopt' will call the
	function specified by VALUE to install the new value.
---

When it now takes two arguments plus an optional argument called
BUFFER-LOCAL that is non-nil when the value is set through
`setopt-local'.





In GNU Emacs 31.0.90 (build 1, x86_64-pc-linux-gnu, GTK+ Version
 3.24.52, cairo version 1.18.4) of 2026-07-01 built on X13
Repository revision: 5dd8fe25c23944cb8ac97d2d54b946c6a394d0fb
Repository branch: emacs-31
System Description: GNOME 50 (Flatpak runtime)

Configured using:
 'configure --prefix=/app --with-gnutls --with-tree-sitter --with-pgtk
 --with-native-compilation=aot --with-modules 'CFLAGS=-O2 -pipe -g
 -Wp,-D_FORTIFY_SOURCE=3 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions
 -fstack-protector-strong -grecord-gcc-switches
 -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection
 -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer '
 'LDFLAGS=-L/app/lib -Wl,-z,relro,-z,now -Wl,--as-needed ''

Configured features:
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LCMS2 LIBSYSTEMD LIBXML2 MODULES NATIVE_COMP NOTIFY INOTIFY PDUMPER PGTK
PNG RSVG SECCOMP SOUND SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS
TREE_SITTER WEBP XIM GTK3 ZLIB

Important settings:
  value of $LC_MONETARY: it_IT.UTF-8
  value of $LC_NUMERIC: it_IT.UTF-8
  value of $LC_TIME: it_IT.UTF-8
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
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  minibuffer-nonselected-mode: t
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  delete-trailing-whitespace-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
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  temp-buffer-resize-mode: t
  hs-minor-mode: t

Load-path shadows:
None found.

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Memory information:
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