completion: add and use --list-cmds=nohelpers

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/git.txt b/Documentation/git.txt
index c423810..820ab77 100644
--- a/Documentation/git.txt
+++ b/Documentation/git.txt
@@ -169,7 +169,8 @@
 	groups are: builtins, parseopt (builtin commands that use
 	parse-options), main (all commands in libexec directory),
 	others (all other commands in `$PATH` that have git- prefix),
-	list-<category> (see categories in command-list.txt)
+	list-<category> (see categories in command-list.txt),
+	nohelpers (exclude helper commands).
 
 GIT COMMANDS
 ------------