bug#39254: 26.3 and 27.0.50; The manual is inconsistent about the <Alt> modifier and C-x 8

[Jorge P. de Morais Neto]

Hi.  The manual section [[info:emacs#Modifier Keys]] says "the standard
key bindings in Emacs do not include any characters with these modifiers"
(referring to <Super>, <Hyper> and <Alt>).  However,
[[info:emacs#Inserting Text]] says "a working Alt key acts like `C-x
8'".  Also, the wording in the latter manual section makes it seem that
any key sequence starting with the `C-x 8' prefix works with <Alt>, but,
according to my testing (with C-x @ a), that does not hold for `C-x 8
RET'.

This occurs on Emacs 26.3 and 27.0.50, both installed via an updated
Guix on Debian buster.

Regards
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