documentation: Clarify documentation for special environment variable "default".

documentation: Clarify documentation for special environment variable "default".

The current documentation for the special environment variable
"default" is confusing and unclear.  This patch attempts to clean it
up.

In particular, the current documentation refers to the "number or
title", but then in the example it gives, the menu entries and
submenus all have numbers *in* their title; furthermore, there is no
example given about how to choose the number, or any indication about
whether counting is zero-indexed or 1-indexed.

Having a cleaner example and presenting all variants (numeric, title,
and id) should make it clearer to the user.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>