Documentation/git-archive.txt: a couple of clarifications.

The description of the option gave impression that there
were several formats available by using three dots. There are
no other formats than tar and gzip currently supported.

Clarify that the archive goes to the standard output.

Signed-off-by: Jari Aalto <jari.aalto@cante.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-archive.txt

@@ -15,7 +15,8 @@ SYNOPSIS
 DESCRIPTION
 -----------
 Creates an archive of the specified format containing the tree
-structure for the named tree.  If <prefix> is specified it is
+structure for the named tree, and writes it out to the standard
+output.  If <prefix> is specified it is
 prepended to the filenames in the archive.
 
 'git-archive' behaves differently when given a tree ID versus when
@@ -31,7 +32,7 @@ OPTIONS
 -------
 
 --format=<fmt>::
-	Format of the resulting archive: 'tar', 'zip'...  The default
+	Format of the resulting archive: 'tar' or 'zip'.  The default
 	is 'tar'.
 
 --list, -l::