bundle doc: rewrite the "DESCRIPTION" section

Rewrite the "DESCRIPTION" section for "git bundle" to start by talking
about what bundles are in general terms, rather than diving directly
into one example of what they might be used for.

This changes documentation that's been substantially the same ever
since the command was added in 2e0afafebd8 (Add git-bundle: move
objects and references by archive, 2007-02-22).

I've split up the DESCRIPTION into that section and a "BUNDLE FORMAT"
section, it briefly discusses the format, but then links to the
technical/bundle-format.txt documentation.

The "the user must specify a basis" part of this is discussed below in
"SPECIFYING REFERENCES", and will be further elaborated on in a
subsequent commit. So I'm removing that part and letting the mention
of "revision exclusions" suffice.

There was a discussion about whether to say anything at all about
"thin packs" here[1]. I think it's good to mention it for the curious
reader willing to read the technical docs, but let's explicitly say
that there's no "thick pack", and that the difference shouldn't
matter.

1. http://lore.kernel.org/git/xmqqk0mbt5rj.fsf@gitster.g

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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