mutt.1: make it clear that the -H option does not use or want a From_ line

I'm going to write up a diff (well, PR) for the manual page and see what you think.

From the "Sending all mail in IMAP folder from command line" thread here:

http://lists.mutt.org/pipermail/mutt-users/Week-of-Mon-20201116/002128.html

raf made this reply:

The man page says:

   ‐H draft Specify  a  draft  file which contains header and body
            to use to send a message.

Aside from saying that the message includes headers, what else would you like it to have said? To me that says "a full message with headers". I'm not sure what else would be useful to know.

Cheers, Cameron Simpson

Just a thought: if it needs to have a From_ mbox message header (or if it needs to not have that), it might be worth mentioning. Even if the mbox message header is optional, it might be worth mentioning that it's optional. And I think there are several variations on the mbox file format (maybe 5?). Which ones, if any, are supported? Nothing's ever obvious or trivial, especially when it comes to email. :-)