help: mention -a and -g option, and 'git help <concept>' usage.

Reword the overall help given at the end of "git help -a/-g" to
mention how to get help on individual commands and concepts.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Philip Oakley

git.c

@@ -13,7 +13,9 @@ const char git_usage_string[] =
 	"           <command> [<args>]";
 
 const char git_more_info_string[] =
-	N_("See 'git help <command>' for more information on a specific command.");
+	N_("'git help -a' and 'git help -g' lists available subcommands and some\n"
+	   "concept guides. See 'git help <command>' or 'git help <concept>'\n"
+	   "to read about a specific subcommand or concept.");
 
 static struct startup_info git_startup_info;
 static int use_pager = -1;