bug#49405: Attribution for printed manual

[Eli Zaretskii]

> From: Glenn Morris <rgm@gnu.org>
> Date: Sun, 04 Jul 2021 15:58:20 -0400
> 
> The printed version of the Emacs manual only lists a single author.
> Ref eg book-spine.texi "by", back.texi "About the Author", etc.
> The manual is the work of many people (according to the git log, rms has
> made one commit to it in the past 12 years).
> So I think it would be nice to at least get an "et al." in
> book-spine.texi and some mention of the existence of other contributors
> in back.texi.

Thanks, fixed.