gitignore doc: add global gitignore to synopsis

The gitignore(5) manpage already documents $XDG_CONFIG_HOME/git/ignore
but it is easy to forget that it exists.  Add a reminder to the
synopsis.

Noticed while looking for a place to put a list of scratch filenames
in the cwd used by one's editor of choice.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitignore.txt

@@ -7,7 +7,7 @@ gitignore - Specifies intentionally untracked files to ignore
 
 SYNOPSIS
 --------
-$GIT_DIR/info/exclude, .gitignore
+$HOME/.config/git/ignore, $GIT_DIR/info/exclude, .gitignore
 
 DESCRIPTION
 -----------