bug#60930: 30.0.50; 24.3 Choosing File Modes section of Emacs manual does not explain dealing with backup files

[Ihor Radchenko <yantar92@posteo.net>]

Hi,

The 24.3 Choosing File Modes section of Emacs manual explains how the
`auto-mode-alist' works, but fail to address an important point about
backup files being open in the major mode used for files with a name
stripped from the backup suffix:

       Fifth—if Emacs still hasn’t found a suitable major mode—it looks at
    the file’s name.  The correspondence between file names and major modes
    is controlled by the variable ‘auto-mode-alist’.  Its value is a list in
    which each element has this form,

    <formal programmatic description of `auto-mode-alist' follows...>

I think that it is worth mentioning the backup file handling entry in
`auto-mode-alist' in the manual itself:

-----------
By default, `auto-mode-alist' contains:

 ("\\.~?[0-9]+\\.[0-9][-.0-9]*~?\\'" nil t)
 ("\\.\\(?:orig\\|in\\|[bB][aA][kK]\\)\\'" nil t)

meaning that backup files (for example, foo.txt~, foo.txt~12,
foo.txt.orig, foo.txt.in, foo.txt.bak) will use major mode as for
foo.txt - the file with backup suffix stripped.
-----

WDYT?

In GNU Emacs 30.0.50 (build 1, x86_64-pc-linux-gnu, cairo version
 1.17.6) of 2023-01-16 built on localhost
Repository revision: bb679244152dddd9949ca065aa6617457f7a7144
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12101006
System Description: Gentoo Linux

-- 
Ihor Radchenko // yantar92,
Org mode contributor,
Learn more about Org mode at <https://orgmode.org/>.
Support Org development at <https://liberapay.com/org-mode>,
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