Document the -d flag to mail(1) better, which comes handy when

you want to see, e.g., sendmail arguments mail(1) will use.

-H is not an independent flag, it's a modifier.  Also explicitly
say that -H will cause mail(1) to exit as soon as it prints the headers.

MFC after:	5 days

usr.bin/mail/mail.1

@@ -42,7 +42,7 @@
 .Nd send and receive mail
 .Sh SYNOPSIS
 .Nm
-.Op Fl EiInv
+.Op Fl dEiInv
 .Op Fl s Ar subject
 .Op Fl c Ar cc-addr
 .Op Fl b Ar bcc-addr
@@ -50,19 +50,18 @@
 .Ar to-addr ...
 .Op Fl Ar sendmail-option ...
 .Nm
-.Op Fl EHiInNv
+.Op Fl dEHiInNv
 .Op Fl F
 .Fl f
 .Op Ar name
 .Nm
-.Op Fl EHiInNv
+.Op Fl dEHiInNv
 .Op Fl F
 .Op Fl u Ar user
 .Nm
+.Op Fl d
 .Fl e
 .Op Fl f Ar name
-.Nm
-.Op Fl H
 .Sh INTRODUCTION
 The
 .Nm
@@ -77,6 +76,11 @@ The following options are available:
 Verbose mode.
 The details of
 delivery are displayed on the user's terminal.
+.It Fl d
+Debugging mode.
+See the
+.Va debug
+mail option for details.
 .It Fl e
 Test for the presence of mail in the (by default, system)
 mailbox.
@@ -84,7 +88,7 @@ An exit status of 0 is returned if
 it has mail; otherwise, an exit status
 of 1 is returned.
 .It Fl H
-Write a header summary only.
+Write a header summary only, then exit.
 .It Fl E
 Do not send messages with an empty body.
 This is useful for piping errors from
@@ -978,6 +982,11 @@ on the command line and causes
 .Nm
 to output all sorts of information useful for debugging
 .Nm .
+In case
+.Nm
+is invoked in this mode to send mail, all preparations
+will be performed and reported about, but the mail will
+not be actually sent.
 Default is
 .Va nodebug .
 .It Va dot