Brief documentation for the mysterious git-am script

The git-am script is nowhere called and nowhere (including itself)
explained, and the name isn't helpful either. For those like me who will
wonder what is it about, add some documentation stub for it to the
documentation.

I probably got something wrong and I don't feel like investigating all the
options - this is just kind of "emergency" docs.

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-am.txt

@@ -0,0 +1,51 @@
+git-am(1)
+================
+
+NAME
+----
+git-am - Apply a series of patches in a mailbox
+
+
+SYNOPSIS
+--------
+'git-am' [--signoff] [--dotest=<dir>] [--utf8] [--3way] <mbox>
+'git-am' [--skip]
+
+DESCRIPTION
+-----------
+Splits mail messages in a mailbox into commit log message,
+authorship information and patches, and applies them to the
+current branch.
+
+This is the replacement for the 'git-applymbox' script.
+Unlike git-applymbox, it can take more than one mailbox file from
+the command line, as well as reading from the standard input when
+'-' is specified. Other differences include changed parameter names
+and less descriptive command name.
+
+
+When initially invoking it, you give it name of the mailbox to crunch.
+The usage hints that it might get interrupted and you will want to
+resume the last round of applying - to do that, pass it no mailbox
+name, and optionally the mysterious '--skip' parameter.
+
+
+SEE ALSO
+--------
+gitlink:git-applymbox[1], gitlink:git-applypatch[1].
+
+
+Author
+------
+Written by Junio C Hamano <junkio@cox.net>
+
+Documentation
+--------------
+Documentation by Petr Baudis, Junio C Hamano and the git-list <git@vger.kernel.org>.
+
+This manual page is a stub. You can help the git documentation by expanding it.
+
+GIT
+---
+Part of the gitlink:git[7] suite
+

Documentation/git-applymbox.txt

@@ -75,7 +75,7 @@ OPTIONS
 
 SEE ALSO
 --------
-gitlink:git-applypatch[1].
+gitlink:git-am[1], gitlink:git-applypatch[1].
 
 
 Author

git-applymbox.sh

@@ -15,6 +15,8 @@
 ## Pay a special attention to the commit log message if you do this and
 ## use a Signoff_file, because applypatch wants to append the sign-off
 ## message to msg-clean every time it is run.
+##
+## git-am is supposed to be the newer and better tool for this job.
 
 . git-sh-setup || die "Not a git archive"