bug#65156: 29.1; Reading from pipe with --insert or insert-file-contents no longer supported

[Eli Zaretskii]

> Date: Fri, 11 Aug 2023 14:45:48 -0700
> Cc: mail@lucaswerkmeister.de, larsi@gnus.org, 65156@debbugs.gnu.org
> From: Paul Eggert <eggert@cs.ucla.edu>
> 
> On 2023-08-11 11:30, Eli Zaretskii wrote:
> > This is fine, but I think instead of "act as if REPLACE were nil" we
> > should explicitly say that the buffer is erased and the file's
> > contents is inserted.
> 
> I'm still a bit lost here. It's news to me that REPLACE being nil means 
> the entire buffer is erased first.

That's actually a bug: it should only erase the accessible portion of
the buffer.  I fixed it now.

> The bigger picture is that I don't know what insert-file-contents is 
> supposed to do in all these complicated circumstances. THat is the 
> various motivations behind insert-file-contents's complex argument 
> combinations don't fully make sense to me. So I'll step aside and let 
> someone more expert fix the doc string, whenever anybody has the time.

Until we wait for someone to figure that out, I attempted to make
things better by fixing the doc string's language as best I could, see
the master branch.

WDYT about backporting this (both the doc string and the erasing fix)
to emacs-29?