git-am/git-mailsplit: correct synopsis for reading from stdin

Invoking git-am or git-mailsplit without mbox or Maildir results in
reading an mbox from stdin.  Mention this in the synopsis and usage
strings.

Signed-off-by: Stephan Beyer <s-beyer@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-am.txt

@@ -12,8 +12,8 @@ SYNOPSIS
 'git am' [--signoff] [--keep] [--utf8 | --no-utf8]
          [--3way] [--interactive] [--binary]
          [--whitespace=<option>] [-C<n>] [-p<n>]
-         <mbox>|<Maildir>...
-'git am' [--skip | --resolved]
+	 [<mbox> | <Maildir>...]
+'git am' (--skip | --resolved)
 
 DESCRIPTION
 -----------

builtin-mailsplit.c

@@ -9,7 +9,7 @@
 #include "path-list.h"
 
 static const char git_mailsplit_usage[] =
-"git-mailsplit [-d<prec>] [-f<n>] [-b] -o<directory> <mbox>|<Maildir>...";
+"git-mailsplit [-d<prec>] [-f<n>] [-b] -o<directory> [<mbox>|<Maildir>...]";
 
 static int is_from_line(const char *line, int len)
 {

git-am.sh

@@ -5,7 +5,7 @@
 SUBDIRECTORY_OK=Yes
 OPTIONS_KEEPDASHDASH=
 OPTIONS_SPEC="\
-git-am [options] <mbox>|<Maildir>...
+git-am [options] [<mbox>|<Maildir>...]
 git-am [options] --resolved
 git-am [options] --skip
 --