Improve sendmail-program doc string

* lisp/mail/sendmail.el (sendmail-program): Document that the
program shouldn't output text (bug#56855).

1 files changed, 3 insertions, 1 deletions

diff --git a/lisp/mail/sendmail.el b/lisp/mail/sendmail.el
index f985b2ceac4..189ad075c47 100644
--- a/lisp/mail/sendmail.el
+++ b/lisp/mail/sendmail.el
@@ -49,7 +49,9 @@
        ((file-exists-p "/usr/lib/sendmail") "/usr/lib/sendmail")
        ((file-exists-p "/usr/ucblib/sendmail") "/usr/ucblib/sendmail")
        (t "sendmail")))
-  "Program used to send messages."
+  "Program used to send messages.
+If the program returns a non-zero error code, or outputs any
+text, sending is considered \"failed\" by Emacs."
   :version "24.1"               ; add executable-find, remove fakemail
   :type 'file)