bug#50658: Error messages including function names violates coding conventions

[Eli Zaretskii]

> From: Stefan Kangas <stefan@marxist.se>
> Date: Mon, 27 Sep 2021 16:25:09 -0700
> Cc: 50658@debbugs.gnu.org
> 
> >> (I guess this would also affect the text we want to put in `(elisp)
> >> Documentation Tips'.)
> >
> > Yes.
> 
> Please find attached a patch.  WDYT?  Too wordy?

No, it's okay.  A few minor nits, though:

> +It is occasionally useful to tell the user where an error originated,
> +even if @code{debug-on-error} is nil.  In such cases, a lower-case
> +Lisp symbol can be added to the error message.  For example, the error
                      ^^^^^
"prepended" is better here.

> +message ``Invalid input'' could be extended say ``some-function:
                                               ^^^
"to say"

> +Invalid input''.  This convention is better avoided in most cases.

That last sentence is unexpected, and seems to contradict everything
you wrote before it.  Which convention should be avoided, and why?