Bug#1032811: Document when --output=FILE won't work

[Dan Jacobson <jidanni@jidanni.org>]

Package: reportbug
Version: 11.6.0
Severity: minor

Man page says:

       -o FILE, --output=FILE
              Instead of sending an email, redirect it to the specified  file‐
              name.

              The  output file is a full dump of the email message, so it con‐
              tains both headers and mail body. If you want to  use  it  as  a
              template  to create a new bug report, see the --resume-saved op‐
              tion.

OK, but there should be some mention of when it won't work:

$ HOME=/dev/null reportbug --template ca-certificates --output=/tmp/z|wc - /tmp/z
Thank you for using reportbug
    127     143    9410 -
wc: /tmp/z: No such file or directory
    127     143    9410 total
$ HOME=/dev/null reportbug ca-certificates --output=/tmp/z --template|wc - /tmp/z
Thank you for using reportbug
    127     143    9410 -
wc: /tmp/z: No such file or directory
    127     143    9410 total

In fact there could even be some STDERR output too, saying what is going on.