mailmap doc + tests: add better examples & test them

Change the mailmap documentation added in 0925ce4d49 (Add map_user()
and clear_mailmap() to mailmap, 2009-02-08) to continue discussing the
Jane/Joe example. I think this makes things a lot less confusing as
we're building up more complex examples using one set of data which
covers all the things we'd like to discuss.

Also add tests to assert that what our documentation says is what's
actually happening. This is mostly (or entirely) covered by existing
tests which I'm not deleting, but having these tests for the synopsis
makes it easier to follow-along while reading the tests & docs.

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

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