bug#70326: 30.0.50; Backup Files documentation does not match observed behaviour

[Markus Triska <triska@metalevel.at>]

In 19.3.2 Backup Files, we read:

   "Emacs makes a backup for a file only the first time the file is saved
    from a buffer."

However, when I launch Emacs with:

    $ emacs -Q

and then do:

    C-x C-w backup.txt RET

then the file is saved from the buffer as expected, and Emacs tells me:

    "Wrote .../backup.txt"

However, even though this is the first time the file is saved from a
buffer, no backup for a file is created, and buffer-backed-up is nil.

If possible, could you please consider rephrasing the documentation so
that it accurately describes the cases where a backup file is created,
or alternatively adapt the behaviour so that it matches the current
documentation?

Thank you and all the best,
Markus

In GNU Emacs 30.0.50 (build 1, x86_64-apple-darwin18.2.0, X toolkit,
 cairo version 1.17.6, Xaw scroll bars) of 2023-09-11 built on
 mac
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description:  Mac OS X 10.14.2