bug#60515: 30.0.50; set-justification-center joins lines before centering

[Eli Zaretskii]

> From: Jean Louis <bugs@gnu.support>
> Date: Tue, 03 Jan 2023 13:56:50 +0300
> 
> 
> When I have following lines in text-mode:
> 
> MY FIRST LINE
> SECOND LINE
> THIRD LINE
> 
> and do M-x set-justification-center
> 
> I get following unexpected circumstance:
> 
>                MY FIRST LINE SECOND LINE THIRD LINE
> 
> Description of the function does not say it is going to join lines. My
> expectation is not that it should join lines but center lines only.

The doc string says:

  Make paragraphs in the region centered.

A paragraph in Emacs is (by default) delimited by empty lines.  So the
above is a single paragraph, and the doc string seems to be correct
and accurate.