bug#73427: 29.4; Doc of `append` not quite complete/correct

[Drew Adams via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

See https://emacs.stackexchange.com/a/82238/105 for a complete
description of the doc bug (omission).

The doc, at least in the Elisp manual, should explicitly mention the
case where all args except the last are nil and the last is a sequence
(e.g. a string) that's not a proper list.

In that particular case the last argument is returned, as is - a
non-list sequence. This is an exception to the otherwise correct
statements that the result is a list (including perhaps a dotted list).

Please read the last part of that StackExchange post, which refers to
this Common Lisp doc as calling out the particular case that's missing
from our doc:

https://lisp-docs.github.io/cl-language-reference/chap-14/be-c-dictionary/append_function


In GNU Emacs 29.4 (build 2, x86_64-w64-mingw32) of 2024-07-05 built on
 AVALON
Windowing system distributor 'Microsoft Corp.', version 10.0.19045
System Description: Microsoft Windows 10 Pro (v10.0.2009.19045.4894)

Configured using:
 'configure --with-modules --without-dbus --with-native-compilation=aot
 --without-compress-install --with-sqlite3 --with-tree-sitter
 CFLAGS=-O2'

Configured features:
ACL GIF GMP GNUTLS HARFBUZZ JPEG JSON LCMS2 LIBXML2 MODULES NATIVE_COMP
NOTIFY W32NOTIFY PDUMPER PNG RSVG SOUND SQLITE3 THREADS TIFF
TOOLKIT_SCROLL_BARS TREE_SITTER WEBP XPM ZLIB

(NATIVE_COMP present but libgccjit not available)