doc: config, tell readers of `git help --config`

The `git help` command gained the ability to list config variables in
3ac68a93fd (help: add --config to list all available config, 2018-05-26)
but failed to tell readers of the config documenation itself.

Provide that cross reference.

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

Documentation/git-config.txt

@@ -71,6 +71,9 @@ codes are:
 
 On success, the command returns the exit code 0.
 
+A list of all available configuration variables can be obtained using the
+`git help --config` command.
+
 [[OPTIONS]]
 OPTIONS
 -------