bug#67638: Description of the variable message-mail-user-agent in Emacs 29.1

[Eli Zaretskii]

> Date: Mon, 04 Dec 2023 23:26:39 +0000
> From: rameiko87@posteo.net
> 
> I would appreciate clarifying this variable.

Thanks, I did that on the emacs-29 branch.

> > Like ‘mail-user-agent’.
> I don't know what this sentence means.

It means you should click the link and read on mail-user-agent, before
you continue reading this doc string.  This is the style we use to
avoid repeating the same information in more than one doc string.

> > Except if it is nil, use Gnus native MUA; if it is t, use 
> > ‘mail-user-agent’.
> Please remove Except from the beginning of the sentence, because 
> non-native English speakers like myself think that "Except if it is nil" 
> means "Whenever not nil". It may be correct, but it's confusing to some 
> people. Perhaps the confusion arises because there's a full stop rather 
> than a comma.
> 
> If the except links the sentence to the one above, then why write "if it 
> is t, use ‘mail-user-agent’ ", which is a repetition at this point? Now 
> it looks from the description like there should be two options, but 
> instead there's five.
> 
> I'm totally at loss!

Here's the new doc string:

    "Your preferred mail composition package when reading email with message.el.
  Like `mail-user-agent' (which see), this specifies the mail-sending
  package you prefer.
  The value can be any value accepted by `mail-user-agent', and in
  addition it can be nil or t.  If the value is nil, use the Gnus native
  Mail User Agent (MUA); if it is t, use the value of `mail-user-agent'."