bug#67639: Description of the variable message-mail-user-agent in Emacs 29.1

[rameiko87]

I would appreciate clarifying this variable.

I have difficulties understanding what the variable 
message-mail-user-agent means and what the options mean. The description 
for this variable doesn't help:

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

> 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!