Update git-config documentation

Documentation/git-config.txt: Added documentation for --system
Documentation/builtin-config.c: Added --system to the short usage

Signed-off-by: Andrew Ruder <andy@aeruder.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/Documentation/git-config.txt b/Documentation/git-config.txt
index c759efb..280ef20 100644
--- a/Documentation/git-config.txt
+++ b/Documentation/git-config.txt
@@ -9,16 +9,16 @@
 SYNOPSIS
 --------
 [verse]
-'git-config' [--global] [type] name [value [value_regex]]
-'git-config' [--global] [type] --add name value
-'git-config' [--global] [type] --replace-all name [value [value_regex]]
-'git-config' [--global] [type] --get name [value_regex]
-'git-config' [--global] [type] --get-all name [value_regex]
-'git-config' [--global] [type] --unset name [value_regex]
-'git-config' [--global] [type] --unset-all name [value_regex]
-'git-config' [--global] [type] --rename-section old_name new_name
-'git-config' [--global] [type] --remove-section name
-'git-config' [--global] -l | --list
+'git-config' [--system | --global] [type] name [value [value_regex]]
+'git-config' [--system | --global] [type] --add name value
+'git-config' [--system | --global] [type] --replace-all name [value [value_regex]]
+'git-config' [--system | --global] [type] --get name [value_regex]
+'git-config' [--system | --global] [type] --get-all name [value_regex]
+'git-config' [--system | --global] [type] --unset name [value_regex]
+'git-config' [--system | --global] [type] --unset-all name [value_regex]
+'git-config' [--system | --global] [type] --rename-section old_name new_name
+'git-config' [--system | --global] [type] --remove-section name
+'git-config' [--system | --global] -l | --list
 
 DESCRIPTION
 -----------
@@ -76,6 +76,10 @@
 --global::
 	Use global ~/.gitconfig file rather than the repository .git/config.
 
+--system::
+	Use system-wide $(prefix)/etc/gitconfig rather than the repository
+	.git/config.
+
 --remove-section::
 	Remove the given section from the configuration file.
 
diff --git a/builtin-config.c b/builtin-config.c
index dfa403b..b2515f7 100644
--- a/builtin-config.c
+++ b/builtin-config.c
@@ -2,7 +2,7 @@
 #include "cache.h"
 
 static const char git_config_set_usage[] =
-"git-config [ --global ] [ --bool | --int ] [--get | --get-all | --get-regexp | --replace-all | --add | --unset | --unset-all] name [value [value_regex]] | --rename-section old_name new_name | --remove-section name | --list";
+"git-config [ --global | --system ] [ --bool | --int ] [--get | --get-all | --get-regexp | --replace-all | --add | --unset | --unset-all] name [value [value_regex]] | --rename-section old_name new_name | --remove-section name | --list";
 
 static char *key;
 static regex_t *key_regexp;