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

[rameiko87]

Dear Eli,

> Oh, and message-setup binds mail-user-agent to a value derived from
> message-mail-user-agent.  Maybe this is the missing piece you were
> looking for.

This may be the key. Let's see if I understand correctly. I would be 
immensely grateful if you could check the "decision-making pseudocode" 
below.

CASE A. We're outside of message.el

In this case check the variable mail-user-agent to decide how to create 
the email. mail-user-agent may say to use something different from the 
Message package, in which case message-mail-user-agent will never 
intervene unless we use Gnus, and it doesn't matter what it's set up to.

CASE B. We're inside of message.el

This may happen either because mail-user-agent is set to the Message 
package, so brings us to use message.el, or because we're inside Gnus 
which always calls the Message package no matter the value of 
mail-user-agent.

Now that we're in message.el, first we look at message-mail-user-agent 
and temporarily ignore mail-user-agent:
CASE B.1 It's the Message package, so send the email this way. Note that 
mail-user-agent does not intervene at all here in B.1, except perhaps to 
choose case B instead of case A.
CASE B.2 It's something else from the Message package and is explicitly 
set: use that whatever it is. Note that also in this case then 
mail-user-agent doesn't matter, except perhaps to choose case B instead 
of case A.
Case B.3 It says to look at the value of mail-user-agent. Then if it's 
the message package, this is the same as case B.1 (except that inside 
B.3 we have used the value of mail-user-agent), otherwise use whatever 
other package mail-user-agent says to use.

Do you think this is correct?