sendmail: document and fix usage of fd #4, fix check for helper failure.

function                                             old     new   delta
packed_usage                                       25663   25694     +31
signal_handler                                       191     215     +24
kill_helper                                           22      31      +9
launch_helper                                        194     184     -10
get_cred_or_die                                      142     129     -13
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/2 up/down: 64/-23)             Total: 41 bytes

1 files changed, 1 insertions, 0 deletions

diff --git a/include/usage.h b/include/usage.h
index 3363e103f..de6f03767 100644
--- a/include/usage.h
+++ b/include/usage.h
@@ -3651,6 +3651,7 @@
      "\n        -H 'prog args'  Run connection helper. E.g. openssl for encryption:" \
      "\n                        -H 'exec openssl s_client -quiet -tls1 -starttls smtp" \
      "\n                                        -connect smtp.gmail.com:25' <email.txt" \
+     "\n                                        [4<username_and_passwd.txt]" \
      "\n        -S server[:port] Server" \
         ) \
         USE_FEATURE_SENDMAIL_MAILXX( \