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

[rameiko87]

Dear Eli,
Please understand that I'm really trying my best to get this variable, 
which remains obscure and mysterious despite my technical proficiency, 
which I consider to be above-average. This is the only variable which I 
currently have a problem with (as far as the Mail group is concerned).

>   "Your preferred package for composing and sending email when using 
> message.el.

I read very attentively your previous message. mail-user-agent is the 
package which is started when, say, I click on a mailto link. In my 
case, this is the Message package, which as far as I understand is 
exactly message.el

>  Like `mail-user-agent' (which see), this specifies the package you 
> prefer to use for composing and sending email messages.

Then, I understand that while we're using message.el, and we're done 
writing the email, we need to send. Is it here that 
message-mail-user-agent intervenes?

>  The value can be anything 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'.

This doesn't check out, even just because mail-user-agent accepts five 
options which are Message, Mail package, MH, Message_withGnusfeatures, 
but instead here we miss the options for "Message package" and 
"Message_withGnusfeatures", so message-mail-user-agent doesn't offer all 
the options that mail-user-agent offers. One explanation for this would 
be that "Message" doesn't help with the task which 
message-mail-user-agent wants to accomplish, therefore "Message" is 
absent from the options and "Message_withGnusfeatures" becomes "Gnus 
native" in the options. But it can't be the case because then a 
contradiction would follow: say I set the value of 
message-mail-user-agent to t, and the value of mail-user-agent is 
"Message package": this is an absurdity because then "Message" would be 
tasked with what message-mail-user-agent wants to accomplish, and we 
excluded this by hypothesis.

And also setting the value of message-mail-user-agent to nil: is it the 
same as setting the value of message-mail-user-agent to t and the value 
of mail-user-agent to Message_withGnusfeatures?