git-bundle doc: update examples

git-bundle doc: update examples

This rewrites the example part of the bundle doucmentation to follow
the suggestion made by Junio during a recent discussion (gmane 108030).

Instead of just showing different ways to create and use bundles in a
disconnected fashion, the rewritten example first shows the simplest
"full cycle" of sneakernet workflow, and then introduces various
variations.

The words are mostly taken from Junio's outline. I only reformatted
them and proofread to make sure the end result flows naturally.

Signed-off-by: Nanako Shiraishi <nanako3@lavabit.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>