[bug-mailutils] Mention how to manipulate messages via real batch job scripts

[積丹尼 Dan Jacobson]

On (info "(mailutils) Specifying Messages")
and many other pages, please add a note that
even the simplest
"p 1"
won't work from scripts, or --exec
because the mailbox hasn't been read in yet.

If one wants to script "p 1" totally via a batch job, one must use e.g.,
$ echo -e p 1\\nx|mail.mailutils #non destructive example