archive: document limitation of tar.umask config setting

The local value of the config variable tar.umask is not passed to the
other side with --remote.  We may want to change that, but for now just
document this fact.

Reported-by: Jacek Masiulaniec <jacek.masiulaniec@gmail.com>
Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-archive.txt

@@ -98,7 +98,8 @@ tar.umask::
 	tar archive entries.  The default is 0002, which turns off the
 	world write bit.  The special value "user" indicates that the
 	archiving user's umask will be used instead.  See umask(2) for
-	details.
+	details.  If `--remote` is used then only the configuration of
+	the remote repository takes effect.
 
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